Accounting on ads.
I think a good feature will be if the user gets an account to buy "virtual" points. The ads should not costs "money" but points.
For example: A user (or the admin) registers for 10$ = 100 points to a user account. A ad cost 5 points and bold 1 point a.s.o..
This gives the ability to buy for a longer term points, and combine this with a "Club" membership. A club-member gets 100 points every month for example :-)