I had a similar difficulty in Spain. A street vendor trying to sell me a shoe shine, but my shoes were not a type that would take a shine. I told him in English, hoping that he would at least understand the tone and the multiple uses of the word "No", but I made a fatal mistake. Showing him my shoes that were not shineable (if that wasn't a word it is now), I said something like "These shoes aren't meant to be shined! See? See?"
He immediately set to shining them.
I suspected he was legitimately retarded - though it's possible he was just crooked (or both) so I had some compassion and let him (uselessly) shine my shoes from that point. He had already told me the price and it wasn't terribly high so I went ahead and paid him, figuring that he must really need the money to be pestering tourists so badly.
But when he complained and told me that that was the price _per shoe_ and I had to pay as much again because he had shined _two_ shoes I told him off and walked away.