All BT are doing is responding in the regulator mandated way, the other provider notifying BT your line is to be migrated, BT advise you , (in case it’s not what you want ) , if it’s the same company each time, you would think they would question the person applying to take your line over , asking why the existing provider keeps cancelling the takeover.
If it’s a genuine mistake , you would hope that the new provider would ask their applicant to double check they have used the correct address , as they are obviously incorrectly selecting your address , if it’s malicious I don’t suppose there is anything BT can do, AFAIK , BT cannot put a blanket , ‘do not migrate this line’ notice on your line , so if it’s malicious and the person messing with you has the stamina, you may have to keep fending them off.
If it is a genuine error , and your address is easy to misinterpret, like a house subdivided into flats , check with others in the building , if they are trying to migrate and keep getting their order cancelled
