Spam, in the simplest sense, is when a user uses a weapon or tool more than is recommended for the most efficiency of the weapon or tool.
What constitutes as spam?
While this is a very thin line with hard to determine boundaries, spam is when a weapon of major destructive power is used in excess of its useful and normal limit.
Any weapon that is used to cause it to reload when no zombies are nearby can be considered spam, especially if someone were to point at the sky and waste all of their ammo.
Weapons that create explosions, such as the Dubstep Gun, should not be used on small amounts of zombies. Explosions can usually take out 6 or more zombies, so any explosive weapon shooting 2 or more rounds every 10 seconds would be considered spam, even more so when only shooting one zombie at a time.
For weapons that create fire, such as the Flaregun or Scatterflame, should only be fired once the previous fires have gone away, or if you are too far away from a fire to even tell if it's still around or not.
The Thundergun is special, due to the fact that it freezes the server in order for it to draw a path (So it can do collateral), gives it special circumstances. If anyone uses the Thundergun as a primary means of defending themselves in close range from zombies, and they use more than 3 shots consecutively could count as spam.
Another way of spamming is spamming buildables, this would be placing many pieces of buildables with no helpful intentions. An example would be to purchase many buildables and place them randomly or in a mega pile of them, or in someone else's base. Not only is this annoying and troublesome, spamming buildables can cause major amounts of lag.
All of these things are bound to change.
In the future, all of these values may change and evolve as the game gets laggier or not.
What happens when I spam?
Well, First off, spamming causes lag, which breaks the rules. Second off, it can ruin the experience for other players. Spamming can lead to games being slow and possibly crashing. If you spam, certain features may break. If you spam, being put on the Disrespectful player list is possible. In all, Do not spam. It ruins the game, and is simply not fun to others who want to play the game the way it was intended and not break the rules.