Best way to disable 12v battery.

For the presets that are kept in volatile memory, keeping the power supplied is the only way not to mess them up. For times when your task is to replace the 12 volt battery, there are ways to do that (attach a temporary 12 volt source somewhere else to keep the memories powered during the swap).

For those times when your task is to cut the power (so you can do something you don't want to do with power supplied), that technique kind of misses the point. For those cases, you just have to cut the power, and accept that your presets will be gone after.