What can I do when the other party refuses to cooperate?

This is a typical problem which parties who co-sign encounter.  The answer lies in what was the agreement as between you and your co-party.  If you have agreed to be mutually liable for a debt, then there is no legal rigth per se to be removed from the debt just because you can no longer get along.  You may wish to seek judicial intervention for a declaratory judgment regarding what each of you are responsible to do in relation to the debt; however such a process is an expensive way to simply resolve a difference of opinion.