Well, lets say your "paid" usergroup is group #5.
(SELECT * FROM w3t_Groups will show you all groups and their numbers)
Then code like this:
First on the page, find the bit that STARTS like this:
$user = $userob -> authenticate("...
Often there are a list of User table fields in the quotes inside the authenticate() function. Add in U_Groups to that list.
Then in your page, code like this (assuming we don't want to show ads to group #5):
if (!strstr($user['U_Groups'],"-5-")) {
// Display your ads here
}
Hope that helps.