Hmmmm... What if next to the payout threshold box will be a PIN Code installation button which you can only press once to set a password. When changing the threshold you will need to enter that PIN Code.
That's one of the ideas we have. However what about people that didn't set their PIN? Someone evil could set a PIN and then the real owner wouldn't be able to payout at all. The only way they could prove they're the owner then would be signing a message using an address, but most people have no idea how to do that (and some people use exchange addresses, where they don't have access to the private key).
I believe that the only sane way is signing messages from the very beginning. That way only real address owner can change the threshold. If the owner can't sign a message, they'll be stuck with the default lowest-possible threshold (which isn't that bad, at least they'll get the coins).