You could just make him hate your guts, then put your spymaster to watch him and discover the plot he will inevitably set in motion. When it is discovered, you can safely give him the boot with minimal consequences. Is it amoral? Yes. Does it work? Usually.
Now, I have a question. My character is the new Queen of Ireland, having inherited the position from her father. He was immensely popular, even after I took the entirety of Ireland under his demense in the last year of his life. Now I've given away all of the Duchies except for Munster, and all of their subordinates, to various cronies who are not related to me. Giving away the duchies gave her a pretty big relations boost as intended, but they're all still mighty pissy that she's a woman who took on more crown authority. I'm dealing with major revolts every couple of years, and I probably would have been overthrown if I wasn't allied with both France and Scotland.
How can I rein these guys in? I've considered removing the land-holding from ducal titles and giving each of the counties an actual count instead of just having the Duke manage them in order to decentralize things (outside of my own holdings, of course) but I'm not sure I can afford to take the hit such tyranny will incur and that will make managing them a lot more difficult. Has anyone else been in a similar position? I posted about this plan earlier, but I didn't count on my son dying of typhus.