A button where if you need to you can block someone you dislike from your site.

Sure, it's in your host control panel and it's called "Ban Ip". Other that that, how can you stop someone from visiting your site?

OR are you asking the same question as the person below? How to block someone from advertising on your site?

Unless the person leaves a nasty comment, I would welcome the traffic!

im not sure why you would want to do either, i can see not wanting a porn site or something advertising on your site.