Bring up the console in game (~ or whatever key you use to open it up) type charinfo and then mouse over the portrait of the char you want the id for (it's easy to spot and you can gain other info on that char as well). Now type into the console kill (id#) without the parentheses of course and BAM! person with that holding you wanted is no more.(Basically MoxthiroXeli was correct)
Hope that works for you. While I've never used that command to kill anyone I have used the charinfo one to quickly change between rulers using the chars id# and a different console command so I can either help them out or accept a marriage request that they normally wouldn't accept. Stuff like that.
Also there is a topic right here called Console Commands and I would have a look at that. It's where I got all of the commands I use, there is another topic with the cheat events in it. Though I am unsure what it is called now and since one of the updates to the game some of the cheat events don't work 100% properly now but are still quite useful.