I see these BMA goons on the streets daily, eyeballing tourists.
The official line is that even if you do drop a fag butt on the floor you don't have to pay.
At least not right away to the official that grabs you/is asking for an inflated amount. Demand to be taken to the nearest police station immediately and start making a call to your embassy. Don't forget to video the entire encounter and take the BMA officials name and shoulder number.
If they start throwing silly numbers at you like 10,000 baht etc call your lawyer as well.
They will soon give up harassing you once you have firmly stood your ground and proven that you are not a mug.