It must be a difficult one to police! DF to find the culprits would be all very well and good in UK airspace, but the majority of the unwanted transmissions I've heard are from people trying to contact their handling agent without switching frequency first - annoying, but surely forgivable.
I think the other end of the Bay of Biscay is more problematic. Countless times I've found myself deselecting box 2 due to chatter in Spanish and hilarious 'Franco' based distractions. I wonder how many people have found a Mirage on their wingtip after Madrid failed to hand them over to Brest, which could have been avoided had it not been for a muted box 2?
A Europe-wide or greater effort to track down and deter the culprits would be great, but sadly I can't see it happening!