Someone keeps removing my “no flyers” sticker and putting ads in my mailbox

I live in an apartment in Tokyo. The mailboxes are inside the building, but the building has no locked entrance, so anyone can walk in.

I put a 無断投函お断り sticker on my mailbox to stop ads and flyers.

Last week I found that someone removed the sticker and put it inside my mailbox, along with some ads. I put the sticker back.

Today it happened again. The sticker was removed and ads were inside the mailbox.

So it looks like the person who delivers the flyers is doing it on purpose.

Has anyone here had something like this happen before?

Is there anything I can do about it? I feel harassed at this point
Should I contact the company on the flyer, building management, or someone else?

I am also thinking about putting a small camera facing the mailbox.

Any advice would help. Thanks.