Stupid emails about returns and charges

For the third time in a few weeks I have had an email telling me I have equipment to return. I do not have any equipment to return. They are threatening me with a £65 charge if I don't return the kit I don't have. I have phoned twice and (I thought) agreed with their representative that the charge would be removed. 

I can't be bothered to phone a third time for the same thing to happen again. 

The reason I think this is happening is that some time ago I thought my version 5 hub was not working. I requested a replacement, only to discover that I had accidentally switched the hub off and in fact, it was perfectly OK. I telephoned and (again, I thought) cancelled the replacement. That's what I was told.

The email specifies the serial number of the hub they want returned, and as I have told them, this is the one I am actually using. I don't have any other ones. How can I stop them sending these threatening emails?