How to remove your Minecraft Forum account
Minecraft Forum uses Curse for its accounts.
To delete your Curse account, go to http://curse.cursesupport.com/tickets/new and select the appropriate option (for a free account, “delete non-premium account”.
Helpful Trick: Removing online data
Helpful Trick: Remove third-party logins
If you signed up for Minecraft Forum using a third-party login such as Facebook or Twitter, you probably want to revoke all access from Minecraft Forum. Click here to find out how.