Option 1: Hang up the mic in a grounded hanger.
Option 2: Get a HM148G, which has a two-part hanger that provides both ground and off-hook sensing to the back of the mic and allows you to mount an ungrounded hanger on plastic.
Option 3: Open the mic and ground pin 8. Re-assembly is a three handed dexterity challenge. Caution: do not confuse pin 8 with pin 1 as #1 has positive voltage and shorting it will release magic factory smoke. DO NOT jump Pin 8 to ground on the front face RJ-45 as you will no longer be able to clone the radio.
Option 4: (My choice) Pop the front face off and put a drop of solder on CP-1. It is easy to locate on the top edge of the board. That will tell the CPU that the mic is always on-hook.
Confirming, there is no F1700-series programming option to set off-hook scan. The radio has been around for 15 years, and the same firmware also does the F70 portables where off-hook is not an issue. Io fix this issue Icom provides option 4, above.