what you do is you get the ban ready to go then before you hit enter you tell them then hit enter. so then they get a little warning or just ban them then deal with them bitching when they get back.
discreetly get the ban command ready, go into spec, get the demo, then do the command? it's not hard. ive had like... 1 or 2 people in the last year leave before I could get the ban off, and even when that happens you have there Steam ID, and a demo, so it doesn't really matter. you can send the demo into vacbans for review, and you can also keep an eye out for that steam ID 2 resurface... which will more then likely happen.
Maynard - The WCS Guy