fn deleteCupFaces =
(
	if selection.count != 0 do
	(
		local delPoly = polyop.deleteFaces, delMesh = meshop.deleteFaces
		for obj in selection do 
		(
			local fC = getNumFaces obj
			if isKindOf obj Editable_Poly then (delPoly obj #{(fC-2)..fC} delIsoVerts:true)
			else if isKindOf obj Editable_Mesh do (delMesh obj #{(fC-3)..fC} delIsoVerts:true ; update obj)
		)
	)
)
deleteCupFaces()

This FN works with EPoly and EMesh objects

DELETE  CAP FACES FROM CONVERTED SPLINE

Maxscript