Just create a 6 digit PIN system for trading, item drops, mail, AH etc or simply when selecting a character you have to enter your PIN and it will prevent theft.
Yeah that works too but can also add verification system where you req app to confirm its you basically like what steam did abit too much work to put in but in overall the best secured way to feel safe
Downfall it require abit time to trade but that's how it usually supposed to mean
Best of the best solution
Every mmo private server ya guys play on change username email and password from every server to keep yourself risk free