Spam bots will always try to register. They basically just input data into any form they can find in hopes it will pull their URL on to your page so they can build trackbacks and score higher in Google rankings.
There are a couple of things you can do to prevent this:
- If you don't need people to register for your site (most don't) just turn off new user registration.
- Install a captcha plugin for your registration form. It won't catch everything, but it should decrease the number of bogus registrations you're seeing. A good one I've heard of is Sabre.
You might also want to read this good article by Lorelle discussing anti-spam techniques with WordPress. It's from 2007, but the tools and tips are still very relevant.
First things first, though, would be to delete these bogus users from your database. All they are is clutter ... if they're not real people, they don't need to be there in the first place.