This is a legitimate concern of theirs, but this is best solved with a .pdf file rather than new software.
Say instead of just you, you and a friend built this project and you both administer the server. Then, one day the friend decides he's going to pursue a new career of refilling ATMs in Antarctica, and will no longer have time to deal with your project. You'll quite obviously deactivate his accounts and revoke and security privileges he has. Whatever steps you would take to deactivate the friends' accounts, write those down and that document becomes the procedure to follow if they need to deactivate your accounts.
It just as easily could be you moving onto a different project and they would have to find someone new to manage the server, which is pretty likely as I doubt you're going to work on this project for the Scouts for the rest of your life. In that event you too would want your own accounts deactivated so if something dodgy happens, you won't be blamed for it.
As the presumed sole admin of the server, if your account is the only that can deactivate yours, then create a new admin account as a standby for this event, write the credentials down on paper, and give the paper with the credentials to the owner to be placed in a safe spot such as a deposit box to be accessed as needed, when the time comes.
You don't need to develop some sort of half-baked kill switch, just write down the steps they would need to take to disallow you access, the credentials to do so, and you're set.