You have entered the twilight zone here - the limitations act makes the debt unenforceable, but they can keep reporting it for 6 years from the date of last payment. Businesses lobbied for the Limitations Act RSO 2002 to reduce liabilities for potential court actions for 2 years. The problem is that they can't get it together to actually sue you in the two years, they lose the ability to do so - I guess that didn't work out so well.
Since in Ontario, banks, utilities, etc are used to having things both ways, amendments to the Limitations Act in 2006 allow extensions in contracts for the limitation period beyond 24 months again - getting rid of the loophole for us "deadbeats" to get out of paying. As advised here, when you pay the debt 2 things happen - you acknowlege your liability and the 6 years for reporting starts again.
There was a class action suit against the Reporting Agencies in Ontario pertaining to reporting on debts which are not legally collectable (ie passed the limitation period) but for some reason this has not made it to judgement yet. It seems at odds with our legal system that negative credit information can be reported about you on a debt which cannot be legally enforced - but thats the current legal position in Ontario and Albert where a 24 month Limitation period exists. Frankly, I believe that if they aren't willing to seek a judgement within 2 years of the last payment there must be something wrong with their case ....
Contrary to the Consumer Protection Act 2002, collectors will lie to you about the process, how things are reported, etc. - the Act in all common law provinces lets the reporting agencies report from the date of last payment - there are no exceptions. Therefore, once it is assigned as a w/o or collection on your credit file your are better off "not" to pay and wait out the 6 years. The only exception is if they agree in writing to remove the listing as an "error" and the reporting agency agrees to do it.
Otherwise you not only pay them the money, admit that you owed them (when often they can't collect), but you prolonged the bad credit for several years. Remember collections agencies are apparently under no obligation to tell the truth or do whats best for you - they collect money on commission and will push the limits of the law every time!