Or, as demonstrated on Atamara, forced expulsion of prominent nobles, council-reshuffling, and subdivision of a realm can be powerful.
We spin off Itau into FR, Echiur and Koshtlom into a new realm, and leave "Asylon" in Via with about 1/3 as many nobles. Then we force everyone of city-lord or duke-rank or higher to leave the realms, and put 6-8 foreign nobles in one of the two parts of Asylon to keep it under control.
There are many options.