Removing switches from 4 way switch
In the right-hand box -- take the red and black that go to the traveler terminals on the switch, and cap them off.  There will be a likely white wire (hopefully tagged black or red) in that same cable that connects to a black wire -- unhook that nut and cap off the white wire.  This disconnects the switch loop entirely.  Then wire the black that was connected to the likely-white (maybe tagged) wire to the line terminal on the Insteon switch.  From there, you can remove the wire from the common terminal on the switch and connect it to the load terminal on the Insteon switch, and connect the neutral terminal on the Insteon switch to the neutral bundle in the box (with a white pigtail if need be).  Ground the Insteon switch, and put everything back together in this box.

The center box can be left as-is.  In the left box, your job is simple -- take the wires off the existing switch and cap them all individually.