A Review Template can be deleted in Review Settings, selecting Actions on the right-hand side of the Template and clicking Delete.
General Settings > Review Settings > Review Templates
However, if the Review Template is currently in a review period, it will not allow you to delete those templates. The reason behind this is because if the Review Period is in action and you delete the Review Template it will cause issues and also delete the responses placed against those templates. So to prevent this issue from occurring, when a Review Template is used in a Review Period, the system will prevent the template from being deleted.