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.