Settings> Accounts & Sync> google> uncheck sync contacts. Then you would have to delete the contacts. I can't think of a quick way to do that right now other than a factory reset.

 

Go into all app hit People> menu> view> uncheck the box that shows google with the specific email address they are linked to.

This won't "delete" them off of the phone, they will just be hidden and not show up in people or anything else.
Not sure if someone calls you if their info will show up if the view is unchecked...