Hello friends,
I had this issue too and find a way to prevent most part of them (I'm talking, principally about the 'blank avatar' ones):
- Add a new Obligatory question in your form sign in.
- Don't make your sign in form the index page of your site, create a Welcome page instead, for example.
- Use WatchDog plugin.
I did it, and now only the smarter bots join to my site, and these ones are rarely.
Good luck ;3