Strange users registering and doing nothing

[photo-mariages]

Hi,

I have every day 1 or 2 "strange" users registering to my gallery. They are "strange" because:

- their usernames all have the same format ie:hwlw332 / 3 to 5 letters and 3 to 5 numbers, all random
- they don't do anything: don't leave comments, don't post, don't rate

What should I do? I'm afraid this could be a spam / hack...

Thanks!

http://photographe-mariages.net/nice/

[Nibbler]

If you think they are automated then use the captcha plugin for registration.