IIRC it's
/ban bl_id ? time reason
/ban 27690 0 -1 Too_hot_for_my_server
Of course, using the /ban command, you can't put spaces in the reason.
You've got the console command alternative though.
ban(blid,clientid,time,reason);
So putting
ban(27690,0,-1,"Too hot for my server");
will ban me permanently and show
"CONSOLE has banned Pie Crust for -1 Minutes for Too hot for my server."