Not saying it's a bad idea, but, there are
three four other ways to do this.
1. Amade's Banhammer. It has fairly long range for a melee item.
2. Administrator Menu->Select Player->Ban->Length, Reason
3. Command: /ban name ID reason length (I might have that order screwed up)
4. There's a console command to ban someone, but I'm not able to remember it at the moment.
The idea's pretty good though.
EDIT: Except for...
I smell accidental bans.
This. Perhaps a prompt that appears upon shooting the player to be banned with a "Are you sure you want to ban this player? Yes / No" type deal. Clicking yes would open another GUI with the ban length / reason, choosing No would obviously close the prompt. Just throwing suggestions around.