I have just run a couple of tests on the IP filter.
- I blocked an address specifically (entered full IP).
- I blocked partial address (used * as last octet, which will block any IPs that match first three octets).
In both instances I was then blocked from registering from the IP address in question.
Are you sure that your problematic user isn't using another IP address to sign-up with and then posting from their usual IP address? Registration Validator only checks IP addresses on registration, not at any other point. So posts, etc. would not be blocked, and using another IP to sign-up would circumvent IP protection.
Unfortunately due to the fact it is easy to change your IP address, this protection measure isn't always effective.