How do I close form if not used?

[malcolmk]

use the DoCmd.Close acForm, "formname"
in the "active" event on the main form so that after selecting your option on form fselct when the next form opens insert the command into that forms "active" event.
so eg. form1 make aselection and call form2, when form2 opens its "active" event closes form1.

[FishVal]

Form couldn't be closed while handling Deactivate event, but it could be closed while handling Timer event.

Private Sub Form_Deactivate()
    Me.TimerInterval = 1
End Sub

Private Sub Form_Timer()
    DoCmd.Close acForm, Me.Name
End Sub

[OldBirdman]

Apparently this does work. Thank you. Closing an already closed form doesn't seem to cause an error, so other causes of activate not resulting from return from fSelect aren't a problem.
I say 'Apparently' because testing the Activate condition is difficult. Any attempt to use the debugging tools cause activate/deactivate to fire as the focus moves between the program forms and the debugger.