If you need to delete reviews from users that are no longer active on your system please follow these steps:

1. Go to Crucible administration
2. Go to Groups
3. Create a group with administration rights
4. Add users to this group
5. Add this group to the Permission schemes on you project.

These users will have rights to edit the review, so it can be deleted via UI.

Alternatively you may use REST API for this.
Just send a DELETE request to http://{HOSTNAME}/{CONTEXT}/rest/api/latest/rest-service/reviews-v1/{ID}.