Send it to Ruger without the DAO hammer installed with a note that says: "I bought this gun used, but it is not working right for some reason."
I doubt they'll ask you any questions and you should have it back with a new hammer pretty quickly.
The only problem with this approach is that there's a slight possibility that they'll return it with a new DAO hammer. If they do, send it back with one of the DAO hammers installed and tell them you want a hammer than you can cock.
Of course you're going to spend some money in shipping...
You may be able to poke around and find one somewhere for sale, but you should be aware that there may be some fitting required.