That is strange. Even after rebooting your computer you still have this other username auto-fill in the UBB fields?
Make sure your browser cache isn't pulling up stored pages (maybe), try clearing the cache too.
The only other thing I can suggest is manually deleting the cookie from the cookie folder on your hard drive.
If the above don't work, you're having a non-normal problem. Please try asking for help in the Post Installation Troubleshooting forum. When asking there, make sure to include the UBB version#, a link to your board, and a detailed description of what you've done so far so you don't get the same suggestions.
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Doug
darksaber.hispeed.comI am altering the deal. Pray I don't alter it any further.
--Darth Vader, The Empire Strikes Back