There isn't much you can do to prove that you are not married. You might need an attorney.
On the other hand, the IRS might be thinking you are Common Law married. This is still relevant in some states.
A common law marriage is one in which the couple lives together for a period of time and holds themselves out to friends, family and the community as "being married," but without ever going through a formal ceremony or getting a marriage license.
Please see this LINK for more information.
