Here's a quick-hack way of doing it.
In your template.php, where ever the HTML that calls the ad is, I would place code that checks the database to see if the member has contributed.
Probably this is a job for SSI.php -- just put a function in the SSI.php that returns a true if the member has contributed and a false otherwise, then use an if statement in your template surrounding the ad code. But I could be wrong, 'cause I'm struggling with something similar on my 1.3 board right now

). Not even sure you can use the SSI.php in the template.