I truly would not worry about it. In fact, why tell them to stop? Yes, there are times where you may want to tell them that they 'can stop' (go to one of Biffle's WBT seminars...all day gestures can sure burn calories!), but when it really comes down to it.....it's beautiful-- THEY'RE IN YOUR SYSTEM! I mean, it's fantastic! So what if they are gesturing to rebel, or if they are doing it unconsciously....the fact is, if they are still mirroring your gestures, it means they are still focusing on you. Not on the Tech Deck in their desk, not on drawing or writing a note, and not on their friends. They're focused on you, and more important, on the class material.
So... I say...hooray!
I often find myself telling students to do something, or asking them to stop doing something...and then I have to step back and ask myself why it matters to me... what's my goal? If, at that moment, your goal is for them to be focused and engaged, then let the quirky fellas be quirky.