How to UNSUBSCRIBE from a forum?

you mean .. "Default Thread Subscription"??

- User CP->Edit Options->Messaging & Notification>Default Thread Subscription Mode
- Select "Do not subscribe"

No I mean the whole forum , when I was a guest a need to subscribe or register .. I want to do the inverse : remove all my stored information in the forum database and all thing .. I don't mean posts "if there is already" but only my info remove them all , how ?

You can't. All you can do is abandon your account. Removing the account is not a feature that is enabled.

Hey Brad,
U haven't told me yet ? .. the problem is I usually subscribe in many forums to just have something and then I want to go off .. sometimes admins take my email to send it ads [about the forum only] but I don't want it .. just I want to complete remove my informations from those forums .. how to ? I don't want to ask those forums admins to do that coz they may not remove all info .. usually I haven't posts in those forums or maybe one at maximum .. if u have any idea please tell me ... and if it could be made in a forum please tell me how maybe those admins left it enabled in other forums ?

No, the feature isn't in vbulletin to delete an account by a user.