user serduchkaverka

This user seems to be registered at a log of Drupal sites but never posts comments or blogs. Today I see this user opening sessions repeatedly, but the sessions are never closed. The IP address comes up unfindable in searches on two different sites. I suspect the user is using the site for sending mail or something, but how do I check this? And how do I prevent this kind of thing -- besides deleting this particular user?

Thanks.