I've explained to them that an unsolicited visit to me at home is likely to ruin their organisation's chances of ever getting any business out of me.
My explanation was a little more complete than this post has been, and was couched in rather less formal terms, but at least I explained to the caller that it was the organisation I was complaining about, rather than the caller as an individual. I've left the caller with the understanding that I don't intend to transfer my custom to the organisation concerned.