Spam Blocking by filtering on username / id

[Plutocrat]

This is probably quite an easy question, but I haven't been able to find the answer. I'm running a server where all the email addresses are in the format "user at domain.com". I've noticed that a large number of fake login attempts use the format "user" eg. reception, service, root, admin. 

Is it possible to prevent any such logins to these email users without an @domain.com? Or maybe ignore them. Or drop them from the logging. 

P.