Henk wrote:but than he took a ball again.. and said ( come on do it again! come on come on!! cant do it now!! can you!!..
You should have said: Yes, to him and then announced to the spectators: Now I will show you a clown! And while saying that, you put a sponge ball in front of his nose

Or you could say to him: What's your name? Hans? Ladies and gentlemen, Hans has a sponge ball in his hand, he's now going to show us all a magic trick! Give him a big round of applause!
...or just go to your next trick and ignore him

Good luck next time Henk!