The term spam can refer to a few things.
- Repeatedly firing a weapon many times in succession, usually from a distance.
- Repeatedly firing without regard for actually explicitly aiming each of your shots.
- Excessive use of in-game voice binds.
- Typing an excessive amount of messages quickly.
The first two may sound similar, but they are quite different in that the first one is a very valid and common tactic among good players, and the second one is generally done by bad players who don't know any better. See below for more info on the first one. The third and fourth are pretty much the same except voice binds make noise, and they are both very closely related to the real-world usage of the word "spam". Since you should be familiar with them, those two won't be discussed here.
Tactical spamming
The tactical form of spamming involves standing at somewhat of a distance and shooting at a target until it is destroy - usually an enemy base asset. One weapon commonly used is the spinfusor, as it has very precise aim and can fairly easily disable/destroy enemy deployed equipment in 1-3 shots. The second most common form of spamming is with the missile launcher. Most people won't use the word "spamming" in reference to the missile launcher, because it is assumed that you are spamming if you are using the missile launcher, because that is the reason for which it exists. You repeatedly fire missiles at a piece of base equipment until it's destroyed. The last form of spamming is the most common, and it involves the mortar. When fired into the air, the mortar can travel up to ~400 meters in an arc before hitting a target. As a result, long-range mortar spam is a very common tactic for destroying any piece of equipment outside. It can also be used to clear defenders away from a target such as the enemy flag, as any light armor in the radius of the explosion will die instantly, and other armors won't last more than one or two more shots.
Precision spamming and spam points
At some point a couple years (possibly sooner) after the game was released, a method was developed to allow knowledgeable players to spam with amazing accuracy from great distance at targets that weren't supposed to be possible. This method involved marking a location on the terrain where the spammer would stand, and then marking an exact location in the sky where the spammer would aim his mortar. The points were set up so that players could stand in remote locations and spam targets from the farthest possible distance. Additionally, many bases had incredibly small openings - such as windows - that lead into rooms with important pieces of equipment. This precision spamming allowed players to fire mortars from nearly across the map and into these tiny openings in the base, often destroying inventory stations or generators. If even the tiniest path led into an enemy base, someone probably developed spam points that allowed him to fire mortars through it from out of sight.
High arc versus low arc spamming was also an important line that was crossed with these scripts. Those of you remotely familiar with projectile physics know that if you aim at an angle of 45° with the horizontal, your projectile will travel the maximum possible distance. Furthermore, common sense would then tell you that any distance you can hit by aiming between 0° and 45° (the low arc), you can also hit by aiming between 45° and 90° (the high arc). With low arc shots, you can see your target. With high arc, you generally can't. However, with low arc, you are blocked by walls. With high arc, you generally aren't. Since you can't see your target, among other reasons, high arc spamming was incredibly difficult without memorizing ahead of time where you need to aim. The methods developed for precision spamming made high arc mortar shots much more common, allowing mortars to be angled into crevasses previously though impossible.
Eventually, scripts were developed to make this process easier. A beacon or similar aiming guide would be placed on the target, the player would go stand somewhere and fire mortars at it until he had found a location that he liked. Then, the script would save his standing location as a waypoint, and mark a waypoint in the sky for him to aim at. Often these scripts would allow for convenient waypoint cycling, and generally make it a very simple process for any player to undertake. These waypoints were referred to as "spam points" or "sky-points".
While many people thought this was a very unbalanced tactic, no one wanted to restrict it because they didn't want to exclude the possibility of a player actually using his memory and good aim to make these shots. There was literally no way to tell if someone was using sky-points to mark everything for him, or if he was just that good at spamming and had a good memory. Eventually, however, the DefenseTurret anti-cheat program was released, and it had a variable that allowed for the limitation of waypoints being placed in the sky. This stopped almost all of the script-based methods of precision spamming, but many players found other ways. One way that existed before waypoints was editing the sky texture of a map to place an awkwardly colored pixel to show you where to aim, and simply manually memorizing where to stand. This could also be done without editing pixels at all if you were familiar with the sky texture and could memorized the location based on the clouds.
Another method, used by Silverspirit of the team Aurora in the very first match after sky-points were removed, was by creating a player skin that had marked pixels on the back. If you memorize where to stand, then go into third-person view, your reticle would be over your player model, and you could line it up with the pixels on the back of your player skin.
Eventually, however, most of these spam methods died out.
Combat spamming
Combat spamming is the term we will use to describe the second definition given at the top. Most of the time, this is done by people who can't aim very well. They will aim in their target's general direction and shoot as many shots as the can, hoping that some of the shots will do damage. For most weapons, this is a terrible idea and will always result in you getting killed by someone better than you. The two exceptions are the chaingun and grenade launcher. Chaingun spamming is referred to as "whoring". Grenade launcher spamming is a valid tactic because the power, firing rate, and blast radius of grenades makes it incredibly effective when spammed, especially indoors. Grenades were never meant to by precision weapons, so spamming is actually the correct way to use them in most situations.