The term Spam originally is the name of a canned meat product. Based on its inflationary usage in a Monty Python sketch it was not only adopted for junk e-mail, but also by gamers for text flood in chats and reckless or deliberate overuse of certain weapons in games. Also, entire maps can be considered "spammy" due to their design, which encourages use of "spam" weapons.
Spam Weapons
Many projectile weapons can be abused for spamming. Especially the Bio Rifle, Ripper and Flak Cannon and the grenade modes of the Rocket Launcher and Assault Rifle are considered "spam weapons" because their projectiles can either bounce around corners in sometimes unpredictable ways or otherwise stay around for a longer period of time.
The initial number of grenades in the Assault Rifle and the ability to release them in quick succession prompted the competitive community to limit the initial number of grenades to one with the Spawn Grenades mutator, which was later also included in UTComp. Before this, it was common practice in competitive 4on4 Team Deathmatches to deny the opponent access to a room with important power-ups by concentrating grenade spam on the room's entrances.
The name SPMA for the artillery vehicle is also pronounced "spamma", which is probably intended by Epic. The SPMA submunition's wide spread often makes spamming the target area the only useful thing to do.
Tactical Spam
Even in competitive 1on1 matches, it is common practice to fire rockets, flak shells or shock combos around corners without actually seeing an enemy. This kind of tactical spam is also called "prediction shots" because is is usually used if it's likely that the opponent is in that area. It is not rare that such a shot actually hits the opponent. Especially prediction combos and triple rockets can cause a lot of "surprise damage" and often require the player on the receiving end to retreat because his or her chances for winning an infight just dropped dramatically.
Spam Accusations
Often players are accused of spamming by their opponent out of frustration. Occasionally players are even accused of Lightning Gun "spam", which of course is ridiculous and often a sign of resignation to the opponent's success in the match.