Is it the PGP Notifier (usually seen in the lower right corner of your display) that you are seeing that you want to do away with?
It sounds like you only want encryption and/or signing done when you select this from the Outlook PGP buttons?
If my above above thinking is correct, you can accomplish this by opening PGP Desktop, clicking Edit Policies on the right hand side, and then removing the check that activates the Opportunistic Encryption policy, and then clicking the Done button on the right.