You should find out what the laws are regarding collection of debts. In Alberta, a creditor must start a claim within 2 years of it falling in arrears, or the creditor cannot proceed to judgment. In your case, I would suggest that you find you if they have proceeded to judgment. Find out through your consumer and corporate affairs government office what the time limitations are on commencing legal action.
If they have not met the time limit, and do not have a judgment, the only thing that they can do is yell at you.
If the debt is genuine, they have likely placed the bad credit at the credit bureau and it will stay there for 7 years from the last activity, at which time it falls off.
I certainly would suggest that when they phone, you ask the person for their name, spelling, who they work for, where the office is, full address for the office, including head office, who their supervisor is, and ask them if they have lots of money, as you are suing for harassment. In Alberta, you can file a small debts claim for $25,000, for a fee of $200. and let a judge decide if you get compensation. You should also record all phone calls as to what was said, (tell them you are recording the call on a tape player) as you will be asked to give evidence in court.