I need to know how to be able to leave my forum metafaction. I know there's a way how to, but I just can't seem to find it.

Beneath the EFU website links and header there are 6 forum specific links: FAQ, Search, Memberlist, Usergroups, Profile, and the one pertaining to your PM inbox.

The one you're after is 'Usergroups', click it and it should display a list of the usergroups that your account is tied to. Select the usergroup in question, and hit the 'View Information' button. On that page, there should be another option reading 'Unsubscribe'.