Stop Forum Spam is the built in feature (as of UBB.threads v7.5.8) which checks user IP addresses against a centralized database of IP Addresses. IP Addresses are given out by an ISP, and aren't always static; dynamic IP addresses can change daily, monthly, whenever you restart your modem, whenever your ISP wants it to, etc.
If a legitimate user is being flagged you can either:
1. Disable Stop Forum Spam (which I do not recommend, ever)
2.
Follow the removal guidelines from Stop Forum Spamin addition, some "users" which complain that they cannot login to some sites, and are directed to remove themselves from the Stop Forum Spam website, tend to be human spammers (in my experience).