A subpoena is enforceable by the Court. You are entitled to tell the court that you wish to avail yourself of your spousal privilege and the court may permit you to not testify. However, you need to go to court or the court may issue a body attachment which the Marhall's Service will enforce. You may wish to consult an attorney to discuss your options.
A subpoena is a court order for you to do as it says when and how it says to do it. It doesn't happen in every case, but your failure to obey the subpoena could result in a warrant being issued for your arrest and/or contempt of court proceedings being initiated against you, both of which could result in you going to jail if you blow off the subpoena.