Not the parent or agreeing/disagreeing with them, but to your question: if you get creative, there are a lot of things you could do, some more unorthodox than others.

Tongue-in-cheek example, just to get the point across: instead of calling it Developer Mode, call it "Scam mode (dangerous)". Require pressing a button that says "Someone might be scamming me right now." Then require the user to type (not paste) in a long sentence like "STOP! DO NOT CONTINUE IF SOMEONE IS TELLING YOU TO DO THIS! THIS IS A SCAM!"... you get the idea. Maybe ask them to type in some Linux command with special symbols to find the contents of some file with a random name. Then require a reboot for good measure and maybe require typing in another bit of text like "If a stranger told me to do this, it's a scam." Basically, make it as ridiculous and obnoxious as possible so that the message gets across loud and clear to anybody who doesn't know what they're doing.