What can I do if I don't want to file joint taxes with my ex husband?

Hello, I just went through a divorce with a really dishonest person. He has his own business and I found out the night before he left that he had secret bank accounts and was doing all kinds of shady stuff and of course lying to me. In the divorce, he hid assets and money and pretty much won because I didn't have enough money to keep paying my attorney to fight him. When it was tax time (during our divorce), I told him and my attorney that under no circumstances would I sign a tax return with him (since he cannot be trusted). I spoke to my tax preparer and he said that I didn't have to file with him even though we were married that year. I thought everything was fine and that my ex filled "married filling separately" (this was for 2012 taxes). He just told me today on the phone that he needs my signature on the 2012 tax return and that he had filed jointly (without my consent). He said that "he didn't know" that he needed my signature and pretended like I didn't tell him no on filing jointly. He said now the federal government won't accept any more of his payments until I sign. How could he have filed jointly without my consent and signature and what are my options? Thank you in advance.