(1) Click on the smaller character portrait in the event window to open the diplomacy panel of the plotting character.
(2) Click on "Imprison" to imprison the character. If successful, it will cancel the plot. If unsuccessful, the target will flee and declare war on you.
(3) Click on "Assassinate" and send a welcome party to his next plot meeting. Dead men don't plot.
In addition, you can try to edit the plot requirements to make the AI less likely (objectives.txt in CK2/common)
(2) Click on "Imprison" to imprison the character. If successful, it will cancel the plot. If unsuccessful, the target will flee and declare war on you.
(3) Click on "Assassinate" and send a welcome party to his next plot meeting. Dead men don't plot.
In addition, you can try to edit the plot requirements to make the AI less likely (objectives.txt in CK2/common)