Opening a door is not an invitation for them to come in! they just use the foot in the door tosh!
I had the previous owner of my house leave, then we had the Customs & Excise, Police, baliffs, Investigators all nocking, I told them 100 times he left, that carried on for 3 years, I finally started saying to them Get off my property, even to customs, Big burly debt collectors and Investigators, they were harrasing me for information I did not have, I said I would sue them if they carried on.. or remove them from my property by any means, never heard anything since. Since found out the old owner had over £200,000 of debt and lots of bad dealings. My tip, he helpful but if they persist, remove them from your propery and threaten to sue!