Personally, I'd just ignore the letter. BUT. . . don't say that's what you're doing here or anywhere. Just quietly go about your business. Nothing to see here, thank you. Heck, lots of folks do it this way. Not mentioning any names, of course. Some folks are refusing to participate in many "government" programs that are intrusive (i.e. certain census people). Some folks think that quiet civil disobedience is more effective than getting in some beaurocrats (how do you spell that anyway?) face. Making it personal is unnecessary. Again, not mentioning any names here. And some folks just don't feel comfortable giving out any personal information out over the phone or to other folks that knock on the door. However, should more of these other folks show up at the door again, friends from newspapers will be called and the folks will be escorted off of the property. Some folks have been apologized to for having their lives disrupted.
Some folks are wishing you the best of luck with this unfortunate problem,
Some folks