A Smart-Arse-Tax is a form of punishment imposed on smart-arses by taking money/ other items of value from friends, peers etc when they demonstrate their ability to be a smart-arse.
Calling someone a smart arse is intended to pull them down a peg or two for adopting a position of superiority, especially intellectual:
(1) through sarcasm,
(2) by using obscure knowledge to score points off other people, or
(3) just being too f**king smug when they're right about something.
Imposing a Smart-Arse-Tax, actually fines them for this behaviour, giving the recipient a sense of achievement and control.