when they're kids just shove them in the dungeon, it only costs 20 tyranny, which goes away in 10years, for vassals, and -30 opinion for all dynasty members. (somewhat reliable but risky) Imprison them and shove them in the dungeon (or execute if you REALLY want your vassals and family to hate you). The other stuff is uncertain or risky at best: There's only one surefire way: Disinheriting them via decision, but it costs renown (which eventually no longer is an issue when you make 10+ a month via marrying your daughters to heirs of distant empires/kingdoms). IE: If you conquer most of france, the 2nd heir will become the king of france even if that title doesn't exist, unless there is another real king of france outside of the realm keeping him from creating that title when you die. Just be careful, confederate partition creates titles and splits realms if you have enough land for that. I Think catholics can order some of them to take the vows and become priests too.Д«e careful, unless you disinherit him, if he has kids even if he dies the kid will retain the sucession rights.ДЄnother thing you can do is holy war some duchies and give everything you get to your other kids, so they will have their share before sucession and not take anything from the main kid. Your only choices are probably disinherit or have them fight as knights in small armies meant to lose battles over and over again, sadistic people can plot against their own children as well, and if it's absolutely necessary and you don't care about people not hating you, you can always imprison and execute for massive tyranny. Matrilinial marriage just makes it so all the kids will belong to the wife's dynasty, so that son became a dead end for your family.

Ck2 how to get rid of heir