In MIDI ENABLE, turn all channels (01B to 16B) to off
"MIDI Enable lets you turn each MIDI channel on and off independently when in Multi mode. This feature is helpful when you have other devices connected to the same MIDI line and do not want the XL-7 unit to respond to the MIDI channels reserved for the other devices."
I think the MIDI IN Block Diagram really helps here.
When the MIDI ENABLE setting is off, the midi in is disconnected from the synth engine--which is what you want, if I understand you correctly.