1. Mobster goes in to vote but doesn't put it in the box - he takes the blank ballot paper outside with him.
2. He fills the vote on the ballot and gives it to the coerced voter 1. He expects a blank ballot back, or else. He has his goons watching the voter throw the real ballot in the box, or else.
3. Using the new blank ballot he goes to coerced voter 2, and the cycle continues.
By having dozens of people watching everything except the actual marking of the ballot. Scrutineers are highly motivated: they want to catch the other candidate cheating.
I've been a scrutineer in a polling room. I could watch and challenge anything except the marking of a ballot. And I did. So did other scrutineers. The odds of the mobster keeping a rotating scheme without detection approach zero with great rapidity, especially since staying in the ballot after casting your vote is not permitted.
1. Mobster goes in to vote but doesn't put it in the box - he takes the blank ballot paper outside with him.
2. He fills the vote on the ballot and gives it to the coerced voter 1. He expects a blank ballot back, or else. He has his goons watching the voter throw the real ballot in the box, or else.
3. Using the new blank ballot he goes to coerced voter 2, and the cycle continues.
Of course this is hard to scale.