How do I kill adventurers before they attack?

Adventurers are tough to deal with, even more so since the latest major patch removed the Assassinate button. Using your Mystikos to up your plot chance helps, placing your Magistros in the court he is in might get you a relations bonus with somebody you can persuade to help the plot along. Bribes (did I say bribes? I meant gifts) can push a few people across the line to deciding to help you. I have on occasion managed to marry a high Intrigue daughter or sister into the court the adventurer is from, then invite her to help the plot along also.

Unfortunately, that's often not enough. The adventurers often come from foreign courts, sometimes of differing religions. The relations penalties stack up fast to make the adventurer a lot more likable than the foreigner who wants him dead. At that point it's a relatively uncomplicated (if rather large) matter of gathering the biggest army you can and splattering his ragtag band of miscreants the day it arrives. I suggest mercenaries and lots of them.