Okay, you are in a difficult situation, and you have my sympathy. But I don't think I can offer an opinion that will make you feel better.
It sounds to me as if the debt collection agency played a hardball game of bluff, and you lost.
But you agree that you owe the money, and you did write the cheques. Of course the agency could vary the agreement, but they seem to have decided not to, and you can't force them.
Your life could be made worse if you instructed the bank to dishonour the cheques. If the bank dishonours one or more cheques because you do not have funds in your account then, as Brendan indicated, the debt collection agency has a pretty unstoppable legal claim against you, and their enforcing that claim could cost you even more money.
If you can reorganise your financial affairs, I think you need to look into ways of covering those cheques. Might the bank allow you an overdraft that would help, or can you use a credit union?
PS. Discussion moved on as I was composing my post. A debt is a civil matter, and you can't be jailed for it. Issuing a cheque which you cannot possibly cover might be construed as fraud, which is a criminal matter, but there is no chance of anybody believing that what you did was done with a fraudulent intention. You're not going to jail.