I put all the following code in a PUMP.PRG file

DEFINE CLASS xForm as Form

   DataSession = 2
   oComm = .null.

   PROCEDURE init
      This.oComm = createobject( "mscommlib.MSComm" )
      EVENTHANDLER( this.oComm, NEWOBJECT("commevents", 'pump.prg' ) )
   ENDPROC
ENDDEFINE

DEFINE CLASS commevents AS custom

 IMPLEMENTS DMSCommEvents IN "mscommlib.MSComm"

 procedure Destroy
   ? 'destroy fired'
 ENDPROC
 PROCEDURE DMSCommEvents_OnComm() AS VOID;
  HELPSTRING "Occurs whenever the value of the CommEvent property changes."
 ENDPROC
ENDDEFINE

From the command window, I did the following:

o = NEWOBJECT('xform', 'pump.prg')
o.Show()

I then click the close button the form, the Destroy does fire, the form is released, and I have no dangling private data session. Look for code in your form that may prevent the destroy in the CommEvents class from firing.

Also, please try what I have above to see if it works for you. Then lets procede from that point. It will probably be easier to build from something that does work than from something that is failing.