At least on B&G and I presume the same will be true for most Navico Chartplotters, there is a yellow wire as part or the power connection to the Chartplotter.
This Yellow wire triggers the Chartplotter to turn on, although it does not provide power to the Chartplotter. Its a sense wire that is connected to the NMEA power circuit.
So, if you want to disable the auto-on feature of your Chartplotter you just need to disconnect this wire, you will still be able to turn on your Chartplotter but it will require you to do so at the power button.