Although they are able to state they will pay a visit, they have absolutely no legal right to access or to enter your property without your express invitation... or a formal warrant issued by the court.
I would suggest that you post by recorded delivery a edited (to suit your circumstances) copy of the letter below asking them to forward documented details of this alleged debt, the letter also includes a paragraph and explains that their proposed visit is not welcome.
........................................................ ........................................................
Your name address & Date.
Dear Sir/Madam
Account / ref no.(if known)
You have contacted me/us regarding the account with the above reference number, which you claim is owed by myself/ourselves.
I/we would point out that I/we have no knowledge of any such debt being owed to (insert company name).
I am/we are familiar with the Office of Fair Trading Debt Collection Guidance which states that it unfair to send demands for payment to an individual when it is uncertain that they are the debtor in question.
Any planned visit by your representative(s) to my property is NOT welcome and will be met with a refusal to enter my premises, followed with the request of Police attendance should that be necessary.
I/we would also point out that the OFT say under the Guidance that it is unfair to pursue third parties for payment when they are not liable. In not ceasing collection activity whilst investigating a reasonably queried or disputed debt you are using deceptive/and or unfair methods.
Furthermore ignoring and/or disregarding claims that debts have been settled or are disputed and continuing to make unjustified demands for payment amounts to physical/psychological harassment.
I/we would ask that no further contact be made concerning the above account unless you can provide documented evidence as to my/our liability for the debt in question.
I/we await your written confirmation that this matter is now closed. Otherwise I will have no option but to make a complaint to the trading standards department and consider informing the OFT of your actions.
I/we look forward to your reply within 28 days.
Yours faithfully [print name]