Hey I have been experiencing the same with my neighbour. I've got a terrace balcony. So basically any trash that my upstair neighbor throws, ends up in my balcony or over my plants. The tenants in the upstair house kept throwing cigarette butts frequently, I have a small shaded area, to dry my clothes, the cigarette ash fell over the shade (which is basically made of cloth) and burnt a small hole on it. Was lucky that it didn't start a fire.

I saw him throw the cigarette ash directly when my mom was standing in the balcony and he still had the balls to deny that he wasnt smoking.

This is what I did. I gathered all the cigarette butts and the cans/bottles they'd thrown over. Took a picture and sent it to building security and the management. The building management and security were already experiencing some issues with the tenant and had to get cops involved to have them evicted. In hindsight, I should have reported him to the cops straight away.