My opinion is that these agreements aren't binding and are hard to enforce. Theoretically, they can either (1) start a civil lawsuit to seek damages from you, or (2) go to small claims tribunal to make a claim from you.

To do (1), the legal fees will be exorbitant compared to the monetary damages they can possibly win from you, and they may not win. You need not worry about a getting a lawyer -- you can defend against yourself should the agency be stupid enough to sue you. And then please drag the process out to make sure the only winner is their lawyer. I'm just joking -- given that all these agencies are super money-conscious, they will not even want to get a lawyer to send you a demand letter. Speaking of demand letters, all you need is to send a simple reply stating "I reject your claims" and the agency will have to pay their lawyer one more time to send another letter.

This leaves us with (2). The small claims tribunal mediates disputes that involve claims of not more than $10k. My guess is that you can still try to show that the agreement is not fair and you may even attempt to counter-claim against the agency. Never back down!