This question already has an answer here:
PHB states "An incapacitated creature can’t take actions or reactions." (emphasis mine)
Suppose I am fighting a creature that has a multiattack, and I have an ability that can incapacitate a creature as a reaction. If the creature has completed the first attack in its multiattack, could I prevent the remainder of its attacks with my ability? Or is it too late, since the remaining attacks aren't actions?