I can't find anything in the book that would allow me to prevent an enemy from using somatic components. Is there such a way to accomplish this?
I originally thought that Manacles would do this, but they don't seem to actually have any mechanical consequences listed.
The somatic component states:
If a spell requires a somatic component, the caster must have free use of at least one hand to perform these gestures.
Just make sure both their hands are tied and you're set. Or cut both of their hands off. The point is that you can restrict their somatic component by making sure they don't have "free use of at least one hand".
What exactly falls within the realm of not having "free use of at least one hand" is mostly left up to the DM. If they think being bound in manacles doesn't stop a caster from casting with the somatic component then you'd just have to bind their hands in another way.
If you can afford to mutilate them, removal of both hands at the wrist is more than adequate. If not, binding the two hands together tightly will work.
Another option is to imbed their hands in something. There are several spells which do this.
Paralytic poisons also work well. If the save is failed, they're unable to act, bonus act, and react, so they cannot cast spells. (I've yet to see a spell with a casting time of movement.)
Any condition which prevents acting blocks bonus action and action casting times inherently. Ritual spells require being able to act as well. Reactions are lost as well in most cases where actions are lost.
