call System.Runtime.InteropServices.Marshal.ReleaseComObject on all object references, and set the refrences to null, call App.Quit(), in the worst case, call GC.Collect() method and see PRB: Office Application Does Not Quit After Automation from Visual Studio .NET Client http://support.microsoft.com/default.aspx?scid=kb;EN-US;317109
PRB: Office Application Does Not Quit After Automation from Visual Studio .NET Client
http://support.microsoft.com/default.aspx?scid=kb;EN-US;317109