The problem is that every website does the same thing, and now it's impossible to use the internet anonymously. But actual spammers can spend a few bucks on IP rotating services. IP discrimination causes far more harm than good.
Actually I don't think Tor disables javascript by default anymore, but even when I do disable it I still see ads.
This is probably different for larger sites of course but on our scale there's no worries blocking Tor
Edit: Although thinking about it I don't know any ads that aren't served up without some form of Javascript