At this time, there is no functionality that allows you to clear out multiple Coaching Activity Approvals at once.
If a Coaching Activity is assigned to multiple reviewer the submission will stay in the Activity Reviews until the Reviewer has completed the review.
If a submission has already been reviewed by one of the coaches, you cannot remove the submission from your list of 'To Review'.
To avoid this we recommend assigning a Coach or Manager to the User and use the Options under Assign Reviewers as shown below. If one of these options is selected Additional Reviewers do not need to be assigned to the activity
If the Coaching Activity is a Course and the Course's completion criteria only use the first review received additional reviewers are not required to review the submission.
To completely remove the submission from showing in the Activity Reviews tab the course would need to be made inactive or deleted in order for the pending reviews to not show in the Activity Reviewer's view or the Reviewer would need to be removed as a reviewer from that activity.
The image below shows the review has 5 reviews that require attention while there are 15 Activity this reviewer is assigned to.