How do I stop EE sending me letters for the previous resident (>8yrs!)?

So I’ve lived in my current property for nearly 8 years. I’m still getting paper bills from EE for the guy who move out well before then (before the last resident!) mobile phone, tbh I think they’ve actually got a new phone/ contract but still paying EE for their old one. I’ve been ‘return to sender’ these for most of the 8 years (other than a couple of years when I was bored of doing it). Yet still they come. There seems to be no way of contacting them as all the routes seem to require a mobile number to access the service. I had similar issues with HBOS (but you can speak to them, and I got a £150 cheque for the hassle and raising data protection issues). So what can i do to stop it?!