ip block is kind of pointless -most internet providers use dynamic ip addresses.
So to change you just reboot your router and you're unblocked.
If you block whole swathes of ip addresses you will block genuine members.

Pretty much the only way to avoid spam is for either an approval system (which we sort of have in the 'moderated posts' setup) or - what i do on slinging.org - manual signup.
People email me and I create the user accounts. Been doing it for 4 years or so and in that time we've had one single spam post. And that's because I thought we had out first indonesian member. My fault :-)
He didn't last long.

Given how much revenue has to be generated by advertising round here - I'm sure they could pay me enough to make it worth my to do manual sign ups :-)