Enforcement-free English only

I can either be teacher or "cop", not both.  Alfie Kohn points out that one major flaw with a system of rewards and punishments is that if it's to be enforced, you have to be vigilant.  That cuts into my teaching time.  If I have one student with a legitimate question and one student speaking his mother tongue instead of English, I don't want to be diving my attention.

Fortunately, an idea a friend gave me a long time ago got buried in my subconcious and finally resurfaced the other day. 
I have a little doll that's supoosed to look like Slash, the rock guitarist (a long-time crush of mine).  If I hear someone speaking Spanish, I simply put Slash on that student's desk.  Then I step back.  If that student hears someone else speak Spanish, he gives the doll to her.  Whoever has Slash at the end of the period loses a point off their final grade. 
Now I don't have to be cop.

Of, course, they're now policing each other, and as we all saw in the Harry Potter stories, that's not exactly a sweet thing either...