Actually, I believe that you can refuse the search at any time during the process. They cannot arrest you for refusing the search. However, at the point you revoke your consent, you must leave the screening area into the non sterile area at once. A screener or LEO should escort you out of the area. Now I don't know what happens to your bags at that time. I would assume you forefit your rights to the bags, or have to claim them from the airport later. It is my understanding that your bags are in the possession of TSA until they release them back to you at the end of the search or X-ray belt.

The consequences of refusing the search are probably not worth the effort. I would believe that you could not travel that day, even if you went through another checkpoint, as I am sure there would be some notation on the BP. Also I would worry about the possibilty of ending up on the no-fly list, just because they could.

Of course this is just my opinion, I could be wrong.