There's several reasons to believe Kant would not
- include a duty to prevent another person's unethical actions
OR
- consider preventing the action of another person in general moral
First let's start with the distinction between these categories. Type 1 is whether this would fall under the required duties that arise out of the categorical imperative. While Kant supplies several formulations, the important point here is the distinction between perfect duties and imperfect duties (see https://plato.stanford.edu/entries/kant-moral/ and also Marcia Baron, Kantian Ethics almost without Apology). For Kant, perfect duties arise from either
- considering what is universally required
OR
- identifying what is necessary to respect rational beings as ends.
Imperfect duties, in contrast, arise for human rational beings because they at times need things. Thus, we have an imperfect duty to assist others and to develop our talents, because we have at times needed others to act according to maxims that accord with this but not at all times.
To this, we need to join an odd and often over-looked feature of Kant's ethics: that Kant believes we are to assume that humans and other rational creatures act rationally. This is a major motive for his views on lying (prior to the sense in which he thinks it violates a duty to one's self in Metaphysics of Morals).
It's also a mirror image of the Kantian idea that you can do the "right" action but not from the right maxim and thus not have acted morally (Groundwork Section 1 -- discussion of the shopkeeper; n.b. that doesn't mean the shopkeeper acts immorally).
Returning to the question of whether we can have a duty to actively prevent another's unethical actions, we can first ask whether we have a perfect duty to do so. The answer on the Kantian picture is no, because it would make no sense for any rational to have a moral maxim of harming others, so there's no way to have an perfect duty to prevent others from harming since this would never rationally arise.
I want to briefly skip over the potential for an imperfect duty to prevent others from causing harm to state that for Kant, in general, the prevention of the expression of another's will is immoral, because others are assumed to be acting rationally and worthy of respect in our treatment of their wills. This is a strong motive for Kant's view "On a Supposed Right to Lie" in response to a killer where he rejects lying as an option (contra many of his contemporary followers).
At best, I think you could argue we have an imperfect duty -- meaning a duty we can do some of the time -- to prevent others from causing harm. But this would have to be severely qualified. First, it would have to be qualified in the sense that we are not acting to prevent the will of a rational creature, because on Kant's view we should assume a rational creature is trying to act morally. In other words, it would have to be narrowly restricted to obvious wrongs like murder (for which Kant believes there can be no moral maxim).
