Ok, need some BB wisdom!

A frequent visitor to my home is dumping their chewing gum on the ground near the front door. And tonight, I found some inside chucked behind a chair. Apart from the gross factor, this could be a choking hazard for my DS.

I'm 99 per cent certain I know who it is. But even if I knew for sure, I would not want to address them directly about it - they are a teenage boy and I don't want to embarrass them. But how do I communicate that this is not ok? They spend a few minutes sitting outside the front door each week before they come inside. I thought about putting a polite sign there but I don't want other visitors to feel like they are in trouble.

What would you do?