How do I turn off the Zynga notifications from friends on FB, but ONLY those notifications?

Yep. When a notification gets posted to your wall, point your mouse to the right of that post (across from their name). An X will become visible when your mouse is on it, click that X. Then, you get some options, choose ‘Hide [Game]’. You can do this with any app whose posts you don’t want to see anymore.

Yep! Agree! Are you sure you don’t want to send them back a gift, though?
:)

PS Welcome!

or when someone sends an invite just block the application when it comes but hitting block invite from so and so. that way the invite will not come to you and you wont be bothered anymore

Another thing that you could do and that way should change your mind and want to know the game activity is to go to your friends list and click on their pic to go to their page and then follow their post and add comments to their wall.