Username check and validate

Hi,

I would like to add / customize the validation of the user name, so that I can be "reserved" some special username and not give the possibility for users who register to use them. I don't think that i can do with the current functionality, but anyay I can do it by myself, if is possible I would like to have some advice/suggestions on which part of code of easysocial is better to look to insert it.
Thank you very much in advance

Regards