You can refuse to sign the sale papers, and it would be very difficult for him to sell it from under you. In addition the lender may block the sale unless sufficient extra cash is available to settle the full mortgage debt (although they may take the view that something is better than nothing).
I would perhaps also suggest speaking to the lender, and any other parties and making them aware of your inability to cover any shortfall.
However conflict is not a good way of running a business (and a rented property is effectively a small business). I would strongly recommend working towards an agreed solution - why does your ex want to sell? Currently rates are extremely low, but they will rise at some point, and we don't expect significant growth in house prices for some time, so selling may crystallise a loss, but it may be a smaller loss than later.
Sit down with an independent third party (IFA or debt counsellor) and work through your situation - there are lots of different factors which would have an impact.
i dont know if you are still trying to sort this out, but your first port of call should be with the bank/loan provider and with their complaints department.
be certain to take notes of dates, times and contact names so that you can log the events as they unfold.
you can also lodge a complaint to the information commissioner, the following link will take you to the ICO website and their steps in how to proceed further....