Contributing. Not contributing by XX posts in XX time. That means that you would have to change the rules to define exactly how much contributing someone has to do in order to qualify.
Technically, by posting an AD a person has contributed to the site, as ads do draw attention, and ads can get someone on the site a new snake they want, and ads can give someone on the site funds from a sale.
Sorry. You want it enforced in a NEW way, therefor requiring a rewrite of the definition, therefor a change in the rules.
But, hey, nice try.