Hello and thank you for your question.
The following is link for dealing with debt collectors or creditors.
Although a creditor or debt collector can make reasonable inquiries regarding outstanding debts, and may ultimately take legal action through the courts if you do not pay what you owe, they are not entitle to harass or intimidate you and if they do they will expose themselves to having you take legal action against them for unlawful stalking, trespass or other similar offences.
I would suggest you write to them, in similar terms to that referred to in the ACCC link referred to above and advise them they are not welcome to visit your premises. If they continue to arrive uninvited at your door call the police, and explain to them they have been warned off your property and show them the letter telling them they are not welcome, and the police should warn them they are trespassing. If their poor behaviour persists, and the police are not willing to assist (which sadly is sometimes the case) you will need to consider engaging a lawyer to warn them off or sue them on your behalf.
The harassment issues aside, you will still have to pay the debt if it is owed.
I trust the above assists your understanding.
Good luck and please rate my answer.