The idea is simple enough: grant either the Ruler or Judge the ability to name a foreign Noble as an "Enemy of the Realm", and therefore prevent them from joining said realm. Instead of having to ban Nobles that join the realms of their enemies with malicious purpose, it would prevent them from doing so at all.
A few possible caveats that would make it interesting:
-Could only be done once per day (every two turns), or have some time-based restriction.
-The Noble invoking the ban loses points of honor and/or prestige by doing so. The higher the honor and/or prestige of the Noble being declared as an enemy, the greater the loss on the Ruler/Judge who does so.
-Could only be done against Nobles of foreign realms that are 'Peace' or lower in diplomacy, or some other diplomacy-based restriction.
-Could not be done against Diplomats.