spam sign up and preventing them

Hi Chaps
I have a main and sub-site. I only want users to be able to register for the subsite, but when they register there it enters them for both (in database for both sites)?
My main problem, perhaps related, is spam sign-ups. Since you have to enter details, get email and respond - I didn't expect spam users, but I'm getting them.
I'm wondering if, because you sign through subsite, the spam is actually 'hacking' the main site? Or are these spam users common. Would your antisplog help even though users can only browse, forum etc. not create subs/own blogs?
Alas, I'm a bit simple and am not sure what to do - could you offer any advice on preventing spam sign-ups?
Many thanks.

site :
sub (subs=domains) :