Status: Not Being Considered (View Workflow)
Nowadays when a review has comments marked as unresolved and the review author / moderator clicks the Close button this warning is shown:
As can be seen above though, that is just a warning and the author / moderator can proceed with closing the review anyway.
Block the review author / moderator from closing the review if there are unresolved comments.
There are no known workarounds.
You can implement a workflow condition plugin with a condition similar to the following: