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I'm pretty new to xoops, but here is an approach.
Assign all your "subscribers" to be part of some group.
Then, in the either the php or the template (if the ad rotator is in a template) you check if they are in the group or not and if not, don't print the html to cause the ads.
Pretty simple idea, but provided you can get the specific code you'd need, it should work well.
There may be a simpler solution too. I wouldn't know though.
Philip