Spam users, get rid of them

Hello Elggers,

I have developed a social network based on Elgg, it worked good at the begining, but I noticed 2 months ago that many New users had access to the site without any validation test or something similar.  Also, some of their email addresses are not valid, for example: (...@long.mothere.com, ..@api.upheast.com), thus, I think that some of them are just automatically added. 

I don't know how to proceed to fix this out.

Any help will be appreciated.

Thank you.