In every realstic game (arma,rnl,ro2,etc) there will be teamkillers. but i have an idea to lower amount of teamkillers, i got the idea from Heroes and Generals. so this is how it works; if someone teamkills you will be shown who killed and with what, then you have an option where you can forgive or not to forgive the player who teamkilled you. if you choose to forgive it will not count as a teamkill, but if you dont forgive the palyer that will count as the first warning. once the teamkiller gets more then two warnings he will be kicked or banned, (depending the penalty in the server.) I hope you this gives you an idea how to lower the teamkillers.