Not players can't force the AI to make their (the player) kings abdicated. In addition, if your a monarchy you do this, you lose -25 legitimacy (you are after all undermining your own form of government), and if you have a royal marriage and do this, you lose even more. Put in a requirement that monarch need 80 or more legitimacy to use this option, and is can't be abused. This should also give a -100 option penalty with with all allied and/or neighboring monarchies.
Or they could just patch the exploit getting to 100% and surrendering.
Or they could fix the game balance so that getting a random bad ruler doesn't put your nation behind in everything (seriously Paradox, dice roll difficulty is not fun).
I'd also suggest that while Force-Abdicate should be allowed to result in PUs, it should have the same AE as enforcing a PU (which, as I recall, on a nation like France will give you a coalition of basically all of Europe).