Can you clarify your objective here? Do you want to prevent bonus troops from appearing during the game initialisation? If so, you can do this by modding the owning character's record in the history\characters files.
If you want to remove an army present in an existing game, I believe you could do this through editing the save but there are numerous pitfalls around commanders, knights, and any in progress combat. It would be safer to edit the health of raised forces to reduce their stack size down to a minimum, but they will reinforce if you don't wipe them out in game.
It might be possible to scope an event to target a specific realm and demobilise their armies, but I'm not sure if it's possible to scope to a specific stack generated during gameplay.