The OrtekMCE IR protocol sends 3 types of frames:  1) a single frame to indicate button down, 2) one or more frames indicating the button is held, and 3) one frame when the button is released.  Pronto Hex isn't capable of expressing this kind of signal, and IrScrutinizer implements OrtekMCE by sending frames 1 and 2, but can't send frame 3.

Try adding the following to IrpProtocols.ini:
[protocol]
name=OrtekMCE_single
irp={38.6k,480}<1,-1|-1,1>([P=0][P=2],4,-1,D:5,P:2,F:6,C:4,-48m)+{C=3+#D+#P+#F}[D:0..31,F:0..63]

Start IrScrutinizer, and select OrtekMCE_single instead of OrtekMCE as the IR protocol.  The generated signal should send frame 1 and then repeat frame 3.  Perhaps this will keep the receiver from repeating the command.

ETA:  You could also try this irp, which should send exactly one type 1 frame and 1 type 3 frame, if the Pronto remote respects the format.

irp={38.6k,480}<1,-1|-1,1>(4,-1,D:5,0:2,F:6,B:4,-48m,4,-1,D:5,2:2,F:6,C:4,-48m){B=3+#D+#F,C=4+#D+#F}[D:0..31,F:0..63]