How can I pursue legal action to prove a debt is NOT valid?

You have no claim. There is no lawsuit that arises because someone claims you owe them money. You owe me $20.00. See? You have not been harmed by that. What you might have is a defense should they try to sue you for the money. If you dispute the debt, don't pay it. However, unless the debt collector is violating the Fair Debt Collection Practices Act, or similar local law, you have no grounds for suing anyone.