The UK has a 'Digital Services Tax', which is effectively an internet ad tax: https://www.gov.uk/government/publications/introduction-of-t... You could also argue that corporate taxes do tax ads, as they're applied to advertising-based companies, though these taxes usually don't 'target' ad companies. Corporate taxes are passed on to customers, employees, suppliers, or investors; usually one of the first three (and most often the first one), as that list is in increasing order of 'captivity'.

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.

It is actually incentivized in some cases

Ad spend is usually tax deductible