How about this: when you get into the booth you're given an ID. You then cast your vote for Ms A and you can see on the block chain that it was recorded for A. After you cast, the system shows you a bunch of ids on the chain which voted for other candidates and you can memorise the id shown that voted for Mr B. When people come round, you tell them that other id. When they look it up they see that it voted for B and leave you alone.
That doesn't solve the potential problem of vote stuffing... Still thinking about that one.
However in this setup, anyone can bring a smartphone in and take a picture of the screen when it displays anything that separates your id from a fake one.
Sounds like you might be on the right track if you can get over that hurdle somehow.
The smartphone problem exists currently with paper ballots. You're not supposed to be allowed to bring a camera into the voting booth, but this is not enforced particularly well.
One approach to this problem is to make it easy to cancel a previous ballot and submit a new one, so you can get your evidence that you voted the way e.g. your employer wanted you to, but then you can cancel it and vote with your conscience.
In the Brazilian voting machine this is often done. You type the numbers and it loads the candidate info in the screen. Once you click [Confirm] the screen gets blank with the message of success. Therefore the only way to take a picture is before the vote is actually processed. You have a [Reset] button to re-enter the numbers.
As far as I can remember from the instructions last time I voted, in Britain you can do this, just return the ballot to the person handing them out and say that you made a mistake, and they should issue you a replacement.
That doesn't solve the potential problem of vote stuffing... Still thinking about that one.