sm_td_dmgtowarn 20
and you can find here info on how to write book report
// Warning message after this much team damage
sm_td_dmgtokick 50
// Kick and ban after this much team damage. Ban duration is same as sv_vote_kick_ban_duration
// extra
// sm_cvar mp_friendlyfire 0
// Will start block friendlyfire (in L4D2)
Thanks a lot, but is there a possibility to change ban duraton?