How can I leagally avoid paying debts from the 1990';s that my ex refuses to pay. They are all over 7 yrs old

They can only force you to pay the debt (since it is 7 years old) if it is a promissory note debt.
Credit cards are generally considered Open Accounts. Auto loans and other installment agreements are Written Contracts. If there has already been a lawsuit resulting in a judgment, that judgment has a separate Statute Of Limitations, which you can find here.

Statute Of Limitations On Judgements

You are responsible for debt that yo incurred until that debt is paid. If you are a co-signer on the debt then you can only get out of it by bankruptcy.

The first issue to determine is if you personally owe the money. In Ontario, you are not responsible for someone else's debts simply because you were married to them. Have the credit provide you with proof of indebtedness before you do anything. If you are responsible, a debt is a debt. The methods available for collecting that debt change as time passes. If the date of last payment or written acknowledgment is more than six years and you have not previously been sued by the creditor, the debt is likely statute barred. A credit reporting agency will report the debt usually for six years from the date it is written off. Your best option is to tell the creditor or collection agency that the debt is statute barred and they may leave you alone OR offer you a very small settlement. The worst thing you can do is give them a small sum of money or write to them acknowledging the debt. The only payment you should make is the one that settles the debt. Correspond only by phone and make no admissions regarding the debt.