The problem is that there is no up to date list of unused IP address blocks. Nobody will be able to provide you with a list that would make enough difference. IP address blocks are constantly being take in and out of active use. Not to mention the ones that have been assigned to datacenters and other providers, but haven't been re-allocated to their customers. (All it would take is a single as of yet unallocated class-C from some random datacenter in order to bypass your possible filter rules.) Even if there was such a list, IP addresses that were at one time unused will eventually be allocated to new customers, which would leave you blocking access to valid users.