I've also similar problem where several userself register and load tons of blog and bookmarks.

There should be an admin authorization and a mass cancellation.

If you delete the user, all of their content is deleted also

There are a number of anti-spam plugins floating around also.  If you're not using them you probably should be.

There is also a plugin (maybe on github) from Brett or Evan for mass user management.

@Ivan True!

@Matt I had them installed (spam plugins) but didnt activate them cuz i was testing some stuff and in the case of deleting the "user" there was no "user" in my admin dashboard or in the database thats what puzzled me...

@Trajan i will look into that plugin...