Make an int 'hasJustKilled = -1;' and an int 'recentlyKilled = -1;' at the top of client.java.
Then when they kill somebody make 'hasJustKilled = ####;' & 'recentlyKilled = ####;' change '####' to the person that died playerId.
Then make an if command like this 'if(recentlyKilled == hasJustKilled){' and then make it so they don't receive Pk points.