What can I do about this outlet nut?
The lower right hand "ear" on the receptacle can be broken off. It is scored for just that purpose. You bend it back and forth with pliers and it breaks off. The screw will then not prevent the receptacle from fitting flush. If the screw nevertheless interferes with the cover plate fitting flush, you could replace the screw with one which has a lower head.

In fact all 4 of the ears could be removed and the receptacle screwed hard to the metal box. Some sources say that the purpose of those ears is to provide spacer washers in case the box is too far into the wall. So if you want try one or two of them between each mounting tab and the metal box, you could. But this box would appear to be in the plane of the wall and you would save them for later use.

http://www.diychatroom.com/f18/receptacles-mounting-ears-what-purpose-4959/