Here's some suggestions. I know I'm a noob, so please don't throw something at me.

The general idea for dealing with griefers is that the team has to be able to get rid of them effectively, because they spawn there.

These two pertain to tents.
1. When one is at their tent, it should restock, and then wait 3 seconds or something before restocking again. This should help deal with the tent campers who are nigh invincible.
2. The tent should not reload the "current" ammo - the player has to explicitly reload (I said it for SMG-only, but it's probably best if it were for all weapons).

These two pertain to kicking.
3. When one votekicks, there should be an extra tally for the votes from that player's own team, and if either of them pass, the player is kicked. I think this is best put into code; I hope you can read C...

void vote_yes(player_t *p) {
  if(this_ballot != 0 && p->voted_for != this_ballot) {
    p->voted_for = this_ballot;
    ballot_votes++;
    if(p->team == ballot_team) {
      ballot_team_votes++;
    }
    check_votes();
  }
}
void check_votes(void) {
  if( ballot_votes > playercount/2 || ballot_team_votes > team[ballot_team].count/2 ) {
    kick_player_and_end_ballot();
  }
}

4. The player in the lead for their team, provided they have at least 10 points (feel free to tune this value), should be able to kick anyone off their team. This should hopefully deal with the situation where half the team are either griefers or don't votekick, and is based on the general assumption that most people who are simply there to grief generally don't kill people, or don't kill many.
5. When a griefer is kicked from a team, they should be IP-banned from that specific team for half an hour.

And this one pertains to teamkills.
6. People should be able to teamkill people with fewer kills than them, maybe provided that there's a threshold or something. Of course, with that said, you could just permit all friendly fire, which could add to the challenge.

Which of these, if any, are worth implementing? I've played a few flatland games and they are horrible grief-magnets.

Edited by StackOverflow - Made code more readable.