Just add the following code:
FormControl.AfterSubmitPostedData += new SaveFormDataEventHandler(FormControl_AfterSubmitPostedData2);
public void FormControl_AfterSubmitPostedData2(object sender, SaveFormDataEventArgs e)
{
FormControl.Data = null;
}
ps. The code above builds on the alternative xform user control found at this link:
http://www.episerver.com/en/EPiServer_Knowledge_Center/developerforum/2628/6042/6043/
ds.