Contact with disfellowshipped ones considered worthy of the effort at least, is co-ordinated by the BOE and a visit in person is organised, that is, an elder is appointed to come and see you.
The BOE is responsable for their own territory. Are you in the territory from where the letters are coming?
Normally their spy network should have passed on your address so that they should know that you are disfellowshipped. However, if they dont, it would be easy enough in many countries, to get the name of someone living at an address which is constantly a not home, and sending a letter to the householder.
Why not throw it on the fire?