stopping autocreated accounts – Starkman_Nik

I am plauqed with a constant stream of new user accounts on a daily basis. However, these appear to be fake as the name format is indentical. In the past few days I have had the following users created:

Starkman_Nik
Starkman_Mark

I get about 3 different groups of these where the first name is always the same and the second name is changed. Is there any way to stop these automatically created users? I get virtually no spam comments with the plugins I am using but I don’t want the fake users either.