Unknown Users on Game Server

Do any of you ever get cryptic players with player names that consist only of letters (like QRKWCT)?  They have no ping and don't play - but it looks like someone is using our server as a sort of router.  If more than two players join...they disappear.  If you try to query their IP address...they disappear.

Any ideas what this is all about and/or how to block it?

Elite Sniper Corps