I give them this analogy;
'It doesn't really work like that let me give you an example, if you imagine yourself walking down a dark alley, you didn't know someone was following you and that person is about to steal your possessions. The reason it is so easy to take your things from you is because you are unaware its about to happen.
Now if you knew someone was following you, you were carrying a weapon and waited till exactly the right moment to turn around would that person be able to easily get your things from you?'
Let them answer the obvious answer is no.
'Because you are asking me to guess what's inside your head its on your terms, you are prepared for it therefore making it near impossible for me just as described in the earlier'.
One of two things happen here;
The subject will be happy with the explanation and leave.
You have set them up into a mind frame where the ball is back in your court and you are totally justified at this point getting them to write something down or back off with the pressure as you have just explained it will not work if they lock things out.
If the worst comes to the worst and they are still pestering you, tell them to close their eyes turn around and place their feet together. When they have done this whisper in their ear,
'I want you to think of anything in the world, don't say anything out loud and keeping your eyes closed staying in this position. Nod if you have a word, (they nod) now stay exactly still with your eyes closed and not moving till I say the word you are thinking of understand (they nod) it may take a minute or so for me to guess stay absolutely still and do not under no circumstances let anyone pester you in this process'.
Now you have done this wink at their friends, put your fingers on your lips as though to say to his/her friends to be quiet and then walk off. (don't look back)
To the rest of the people watching you whispered something in his ear he now appears stuck to the spot his eyes are still closed and he is not moving at all (you apparently hypnotised a big skeptic).
To the subject he is staying put because he thinks he has to in order for you to guess his word so won't move until you tell him to.
two birds one stone.....Then when he does move (I have seen someone stand for a good 5 mins before) let him claim he wasn't under, his friends will do your arguing for you and ask why the arrogant so and so didn't move and what can he respond
'Because he told me not to?'
and they say you cannot control minds pshhhh!