just need a lil vent!

Many years ago, we bought a ground floor maisonette. We were awaken at 2am by hammering on the door.

It was the police looking for the previous owner, the police had the place surrounded. It was a very frightening experience in the middle of the night.

www.citizensadvice.org.uk/consumer/post/stop-getting-junk-mail/
Not quite what the OP is asking, but may help.
One point is to refer to the Data Protection Act when asking a company to stop sending you mail and to remove your address from their database.

As others have said, sending letters back is what a determined fraudster would do. I once had debt collectors knock at the door of my newly purchased flat, when I showed them the legal paperwork proving that I was now the owner they went away and I heard no more. The people I bought it from had been in a rush to move.......

Not as bad as you, but we have had the same thing with our landline number. We have had the number fir 7 years! The previous owner of the number did not live in this house. Recently a local dentist called to say that the lady had left without paying. We don't know where they live now. They may have innocently forgotten to tell people their number has changed, but they seem to be bad payers.

I finally snapped last year when I recieved yet another batch of marketing letters for an old tenant. I marked each and every once in bold red letters - DEAD, REMOVE FROM DATABASE.
Since then? Not one single letter for her.....!

After over 15 years in this house I think I've finally stopped getting post for the previous owners but there was nothing untoward - they left a forwarding address and I often see the wife anyway as we are members of the same music society and sing in the same choir so passing things on is no problem.

However we left our forwarding address for the people who bought our old house to use if anything arrived after the redirection ended. Sadly they never used it and I realised I'd lost things when they didn't arrive as expected and I remembered I'd not updated my address for the expected items. I really don't see how difficult it is to forward something, even if it is irritating, and receiving something that way would prompt you to update the sender.

I would be suspicious of a seller who didn't leave a forwarding address as I have always left one - but then I have nothing to hide!

My daugher and her partner have tried to register on the electoral roll for my address. I only knew when a letter arrived for her from the local council, as I wasn't expecting anything for anyone but myself I opened it.
The local council said the only reason would be for a credit reference company as they like to check the register and the worse part was if, if she had ticked for it to be private in the same way as ex directory I'd never have known. They were very good and have made a note on my address for future reference. Bad debt is not connected to the address, it is the person and bailiffs are much more reasonable than you think so try not to get stressed over this.

You can take a redirection order on post for up to 4 years! If you haven’t sorted your post by then I don’t know what to say??‍♀️
Royal Mail say this on their website( as you are right that you are not allowed to open post addressed to someone else)

I've received someone else's mail

If you’ve received mail which has your address, but not your name, this is because we deliver to addresses rather than names. If this does happen, you can put a cross through the address and write 'Not known at this address' or 'No longer lives here' and put it back in a letterbox. Where possible, we’ll return the item to the sender hopefully allowing them to update their records.

If you’ve received a ‘Something for you’ card that’s not in your name, we recommend you dispose of the card and once our retention period has ended we'll return the item to the sender.

If you’ve received mail which isn’t in your name or your address, we're really sorry we've made this mistake and inconvenienced you. We'd like to get the item to the intended recipient as soon as possible and would greatly appreciate your help. Please post the item when you can, you won't need to apply any postage, we’ll then process and deliver it to the correct address.