(deleted)
You should be able to click on the "RECEIVE ALERTS" button and choose whether to receive emails for a topic but as I don't use email alerting I have no idea whether it works as there are currently problems with the alerting functionality of the forum
If you wish to leave the forum completly there is an option to delete your account in your settings page.
Be aware that it takes at least 7 days to purge the account and as the other Bob states there have been some other issues with the forum software.
And do not try to sign back in with a new account for at least 10 days to be safe.