I assume you mean some percent of ad spend as a tax? Well, the cheapest ads to run are usually the most obnoxious ones. Taxing ad spend is a bit like taxing you more the nicer the building you build on a piece of land. You're directing the incentive in the wrong direction. A minimum fee per ad run perhaps would have an effect more in line with what you're thinking, I think, though I haven't thought about it much.