Credit reference agencies aren't permitted to blacklist people based solely upon their address.
Creditors can't send in bailiffs until:
(a) they've obtained a County Court Judgement against the debtor; AND
(b) the debtor has defaulted upon that judgement ; AND
(c) the creditor has then obtained another court order, permitting them to use bailiffs.
Since the court processes referred to above require the issuing of court summonses against the debtor, and you've not had anyone trying to deliver a summons at your door, you can't have any bailiffs on their way to your house.
Do NOT simply write 'not known at this address' on the envelope. Instead return it inside another envelope, with a covering letter roughly as follows:
"Sirs,
Take notice that Frederick Bloggs (to whom you keep sending letters) no longer lives at the above address.
This property is now occupied by myself, Agatha Martha Lewis, and my husband, Archibold Ferdinand Lewis. Neither of us know, or have known, Frederick Bloggs and neither of us have ever had any financial connections with him.
Our presence at this address can be confirmed, via credit reference agencies, through consultation of the electoral roll. We have lived at this address since 14 December 2014.
Frederick Bloggs did not leave a forwarding address; we have no knowledge of his whereabouts nor way of contacting him.
Yours faithfully
A. M Lewis"
Send your letter by recorded delivery (or, as Royal Mail now prefer to call it, 'Signed for'). It will obviously cost you a small amount more than simply dropping the wrongly-addressed envelope back into a post box but it's far more likely to get you a positive result.