Picking the reason is the awkward part. Google documents two example reasons for that menu, Spam and Profanity, and doesn't publish the rest. You see the full list only when you open it. Pick the option closest to harassment, even when the review also insults you, because the threat or the address is what the report rests on, not the insult around it. People who do this pick the closest match on the report form and save the detail for the appeal. That's Stefan Somborac, in the Local Search Forum.
And there's a catch. If the report comes back with no policy violation found, Google gives you Google's one-time review appeal. One per review, ever. The appeal names a policy, and naming one that doesn't match your argument can waste the one appeal you have. On this policy the appeal names Harassment and points at the clause the review's own words trip, whether that's the threat, the private detail, the sexual claims about a person, or the call for others to join in. So the filing is the whole job. You file it from your own profile. We never submit anything to Google.
There's a second path, and it's easy to miss. You can also report the reviewer's profile, and Google documents that report only in the Maps app on a phone. You tap the user's name, then More, then Report, and Google's example reasons there are Legal issue and Privacy concerns. Google's separate article on profile reports adds the scope: report a profile only when it contributes content that breaks a policy.