What is a word for annoying behavior which decreases enjoyment for the other players in a game?

Solution 1:

In a tabletop setting, we would call that kind of player "powergamer" (when it's more skillfully using the rules to maximise some effect) or "munchkin" (when it's hurting fun for everyone else).

So I suppose those terms might work; say "don't camp, it's powergaming".

A less games-focused term might simply be "cheap".

Solution 2:

In several online games and metaverses such as Second Life and Minecraft this is known as griefing.

Simply, behaving in a way that causes others grief or upset.

This may not be in the OED yet but it is here.

Solution 3:

poor sport fits well. It is also the opposite of good sport.

someone who exhibits improper behaviour during a game, whether winning or losing

Spawn camping (urbandictionary):

in gaming, when one camps (or remains in one position with the intent to obtain multiple kills) the spawn point (or location where players re-enter the game)

often considered poor sportsmanship because players are often unable to respond in time to fight back, or are caught off guard

If we apply to your example:

Don't camp, that's poor sportsmanship.

Solution 4:

I'd probably call them a saddo, and then maybe a spoilsport.

Spoilsport is quite a loose term, and I would use it simply because they are spoiling the sport.

It should be immediately understood if you said:

"Don't be a spoilsport, dude."

Both killjoy and party pooper could also be substituted for spoilsport.

In response to all of the activity and clarifications, I think the right term for what you're describing is beautifully simple:

"Don't camp, that's sad."

It may be simple, but calling the way somebody is gaming sad, should sting just enough to make them consider a strategy that's less sad.

Solution 5:

If you are happy with the connotations of ungentlemanly or unsportsmanlike, while preferring to keep it gender-neutral and less clumsy, consider unsporting.

This is not online-gaming specific; some of the existing answers (especially griefing) cover that better.