I really do sympathise with you,it must be awful to be persecuted for a condition over which you have no control.
I don't think you can stop anyone from stopping you.
However,what you need to do is to ask your GP,or preferably a Consultant to write a letter (on hospital headed notepaper) stating:~
"The bearer of this letter Mr*****,suffers from a medical condition known as cervical dystonia,which results in the following conditions/symptoms.(Here the Consultant would list the symptoms/condition)
He is not a danger to hinself or others,despite appearances(you might think) to the contrary"
I feel this might help those people that stop you to understand your condition.
As I say they may still stop you,but at least it would finish there.
I hope someone else on AB may have a better suggestion,but I hope mine might be a start.
I wish you all the best,and curse those that judge you solely on appearance!
Alec.