Would this bother you, is it even legal?

[SEOvB]

Originally Posted by TimberFloorAu

The issue in this case isnt copycats or imitators its plaguerism.

Or would seem to be if we saw a screen pic or 2

Its not plagiarism: Plagiarism is the use or close imitation of the language and thoughts of another author and the representation of them as one's own original work.

And to answer the original question, there probably isn't much that can be done, usernames are universal and used on forums all over the world and there is no way to say that for 100% surety they came from your site unless you've some how scoured all forums members lists, compiled the names, and your site is the only one with them

Which if you've done that, it probably wouldn't be worth all the effort required