just be able to justify your actions to law enforcement. you do have the right to defend yourself (within reason).
Seriously though just put the guy in a gift wrap and use your free hand to call the cops if needed.
...unless the cops just disregard your account of the story.
Rape choke.
Edit: Honestly, I would yell help, please stop, please stop as you are pounding his face in.
This, except I would've choked him out had he given me his back. Then I would have quickly left and probably never had to deal with the cops. And if I did have to deal with cops, hopefully the witnesses will identify me as the guy yelling for help.
I LOL'd at the yelling for help part I can totally see someone doing this. As they are sinking in a RNC, yelling help me! with a grin on their face.