the only way ive ever found is cheat, load up the console, and use add_diplomacy ID +30 with ID being the ID number of the guy you want to be emperor. Vote for him to get him in the running and then the AI electors will hopefully vote for him and not you.
Either that or just accept it and pass it on at the first usurpation rebellion but they dont always come.
the game really really needs to have an ability to turn things down. Election, liege given titles and even the new crusade kingdoms. All things that should be accept/decline diplomacy choices are just shoved on you with no choice at all. It doesnt make sense that someone who doesnt want to be emperor would be emperor when hes surrounded by equally worthy candidates who dont want to be and are going out of their way to be.
It really should be set up so that if you vote for someone else then youre out of the running, or have a wee box to tick if you dont want to run or something. Maybe with the option only avialable if you dont have the ambitious trait to fit the theme of the game, but its a thing thats needed or its just silly.