[Abagail]

Well, you can check if the player is wanted, has been killed by the player recently, if they have done any [/me's](assuming it's a roleplay server), etc.

Example:

pawn Code:

stock IsDeathmatch(playerid, targetid)
{
      if(RecentlyKilledBy[playerid] != targetid && GetPlayerWantedLevel(targetid) == 0 && // bla bla
      {
           return 1;
      }
      else return 0;
}

Then you can do something like:

pawn Code:

public OnPlayerTakeDamage(playerid, issuerid, Float:amount, weaponid, bodypart)
{
      if(playerid != issuerid && IsDeathmatch(issuerid, playerid))
      {
             new Float: health;
             GetPlayerHealth(playerid, health+amount);
             SetPlayerHealth(playerid, health);
             CallLocalFunction("OnPlayerDeathmatch", "ddd", playerid, issuerid, weaponid);
             return 1;
       }
       return 1;
}