You have the option right now to not let the actions of someone else turn you into the very thing you dislike. Karma is an absolute bitch, and what goes around, comes around. Clearly you have at least some small voice in your head telling you this is wrong--stop, and listen to it, before you end up doing something you'd never want done to you. Besides, revenge is not going to make the situation better, it's not going to make this guy a better person, he's not all of a sudden going to be so sorry for what he did to you. He will just change his number and move on in a second.
Depends how you look at it. If you see revenge as 2 wrongs don't really make a right then it's wrong to take any sort of revenge.
If you don't care, then go ahead. I presumed he kept harassing you online or kept texting and calling you. But since you blocked him, I think it's time to move one unless he found other ways to continue contacting you and circumvent the block you had placed for him.