How about an opinion penalty for snagging someone from a foreign court? A "Abducted my courtier, -10" penalty for five years should do it, possibly reduced to -5 if the recruiting ruler is the rightful liege. I can't imagine an enemy ruler would be best pleased that you've pilfered all of his courtiers, even if they all hated his guts anyway. The opinion penalty should be further increased to -20 if the invited courtier was a councillor, had a honorary title, or is a family member.
For that matter, recruiting a ruler's direct family member should be a cause for war since you're essentially holding that person's kin hostage, whether with their consent or not. It should provide a casus belli where the aggrieved party demands to have the invited family member imprisoned in their original homeland, and inflict a negative opinion modifier between that family member and the foreign monarch ("Got me imprisoned, -50" for twenty years). The family member can then be dealt with as seen fit through the imprisonment mechanism - if it's just a heir trying to rush things a bit, he can be released, but a treacherous brother can go to the oubliette.
As it is right now, the courtier invitation system is way too broken and abusable since the AI rarely ever makes use of it unless you have a really bad ruler, while the player's court is full of geniuses from around the world.