Problems with unwanted drafts

Maybe just stick a few non-smoking stickers on the walls there? If you only achieve 50% with that, it might already help you a bit. Alternatively: place an ashtray in sight but somewhere else, where the smoke doesn't drift toward you.

I like Tolentino's idea with the ashtray – I might even go further: If space allows, I would set up the cheapest or gifted (classified ads, bulletin boards) terrace chairs/benches/tables in a stealthy night-and-fog operation at a spot within sight where the smoke no longer drifts towards you. It could even become a nice little corner if space permits.

Unfortunately, all such things are immediately disposed of and corrected by the janitor himself, who is extremely nice, but receives a lot of pressure from the Studentenwerk because they conduct regular inspections... unfortunately, it's not that simple :D

You could try whether an activated carbon fleece in front of/in the slots helps a bit. Just a spontaneous DIY idea.