Is there a way to prevent people from signing up with certain usernames? If not, there should be. Someone keeps signing up on my site with my username and a number after it....This could be prevented if there was a way to make a list of protected usernames that a person cannot sign up with

There is already something like this; a censor

Admin Panel > General > Censoring > Username censoring > Add new word

Then make a censored username however you like.

Oops, my bad. I looked around and must've overlooked that