I have been having massive issues as it seems that the list on the website:
http://www.ipdeny.com/ipblocks/
doesn't include a bunch of ranges that I know are valid in Australia.
I used to get maybe one or two false positives a week, not it's more like three or four per day... sometimes more.
In any case at the moment I have added a new zone file xx.zone which I grabbed from here (for Australian in my case):
http://ipinfodb.com/ip_country_block.php#blocklist
I have then added the new XX domain to the "IP address exclusions" list in vBulletin which *should* now add those addresses to the exclusion list.
Now I KNOW that there is going to be cross over between my new XX zone and the original au.zone list from ipdeny. Hopefully this doesn't cause any issues... although I suppose we will see soon enough.
Fingers crossed it helps things.