I do not believe current forum software supports account deletion*. All you can do, is log off that account and log on using credentials for your preferred account.
You can also delete the cookie/login within your browser for that account so it won't, by default, log you on with those credentials.


*This is to protect thread and forum content if someone leaves, as deletion would screw up threads anywhere you've been since joining. This also means a hacker could nuke this forum easily.