All you need to do is create a cookie on his computer when he is banned, and then when he visits the site, you check to see if the cookie exists...remember users can delete their own cookies so cookie banning is no more helpful than IP banning.
Donate to UBBDev today to help aid in Operational, Server and Script Maintenance, and Development costs.
Please also see our parent organization VNC Web Services if you're in the need of a new UBB.threads Install or Upgrade, Site/Server Migrations, or Security and Coding Services.