I had the same as some point and using their website to notify them that you do not need a licence (as advised in the letters) does stop this. After that they only send you one letter every couple of years to ask you to confirm again.
However, I do consider it is harassment to keep sending these aggressive and threatening letters.
BBC claims to have some TV detector technology https://www.youtube.com/watch?v=Vv2ZqZmC7u0