Can an ex-wife who changed her last name when she remarried go back to using her first ex-husband's name after a second divorce?

You can not force someone to change their name if they legally have a right to use it.  Your ex-wife legally acquired your name at the time that you were married.  She probably was given the right to change it to her maiden name after the divorce but she could not be ordered to by the Judge.  The right to change one's name belongs to the individual whose name is being changed. I can understand how frustrating it must be but it is doubtful that you can do anything about it.  I would, though, consult with your divorce attorney to see how things were worded in the decree. Maybe it was done in such a way as to legally change it back. Then you have an argument here. Good luck.