I have before, but since the update fucked it up, i cant anymore.
Its kind of bullshit that you're not allowed to ban someone if they left (for example, when avoiding getting banned). but if they don't. you can.
thats half of what ban requests are for. its much safer and easier just to have zero ban him. that way youre sure the persons banned
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.