[Gandalf Parker]

There are maps that remove all recruitables from every province. Telling a province that it has no recruitables is easier to do in map commands than modding. If you mod to remove the units its alot more work, and there are overlaps with some nations recruitables.

You could wipe out all nations PD in a mod by listing each nation and using the command to set a nations PD units to zero.

Summons might be trickier.

I also wonder why.
Hmmmm.... it would create a game of "dominions" which is much more like other war strategy games. Possibly as a challenge to strategists Or as a stepping stone for new players to learn the game. It would limit the game to fort-created armies marching against each other. Formations, strategies, etc without some of the unique overhead of the game.

You might want to look into some of the "chess game" mods. They tend to remove the nations, fill the empty nation slots (73-95?) with matching nations of fairly capable units. If you added one of Kristoffers research limiting mods (he made one to limit to level 4 and one for level 6). And then went with a No-Independents modified map. Would that get you close to what you want?