Not quite the same thing, but similar. I was in a 100 mile charity race a few month back and while in the fast group a guy came up beside me in a TT bike, in the drops.

I asked him not to ride in the drops (politely) as he had no real control of the bike. His response "I have control and was riding behind people knew". I shook my head and left him behind.

So if they don't listen, do as botto mentioned - drop them. Maybe you can hurt them for awhile before dropping them off the back.