You can start by gently reminding the actor every time they commit the offense that what they're doing is the wrong way, and what you expect to see instead.  If they continue to ignore you, then you need to be more demanding.  All you can do is stop them and correct them, time after time after time after time.  Explain to the actor what they're doing, and why it's inappropriate in a straight play.  Tell them it may "feel" wrong, but it "looks" just right.  See if you can get the offending actor to sit in the audience while you have some other actor demonstrate why what they're doing is wrong.  If you're lucky, the actor will see how stupid it looks. 

This reflects on your position and stature as a director, too.  You must be relentless in pursuit of your expectations, and be prepared to drop the actor if they can't comply.  You're better off with a moderately good actor doing things the right way, than with a slightly better actor doing things the wrong way.

At least, that would be my take on the situation.

Showing by example may just be the answer. Lord knows the gentle reminders, light nagging, then finally physical threats didn't help. Next time...