Use the command "tempbanclient [clientnum]". To find out their number use the command "status".
To set how long the ban should last, use "sv_kickBanTime" (use it before temp banning them). This is in seconds and as far as I know the maximum for this is 3600 (1 hour).Code:Status (you find they're client 5) tempbanclient 5
Hope that helps,