The marmite was deserved. If you’re going to make someone’s life miserable and insecure for six months then we’re going to roll out the red carpet on pettiness.
The best move is to avoid playing the game.
I think the person in question would be in vastly larger amounts of trouble for even raising his head above the cesspool he floated in for a moment. It would be like a chase from the Benny Hill show with local housing enforcement, HMRC, the police and a trail of angry and abused tenants.
The best move is making the game worthless so there are no winners. Shit on the board. It's a stalemate then.
The put the keys through the letterbox thing was actually a masterpiece of idiocy on his part which I refrained from mentioning to him.
Isn't every long-term benefit long-term only in hindsight?
It's honestly best to cut your losses and walk away.
That may be a criminal act in your jurisdiction. My local laws allow the landlord access, but the landlord needs to provide 48h notice of the inspection (with a list of specific exceptions for emergency work, mostly around plumbing and electrical work) and cannot bar the tenant from being present during the inspection.