Allowing reviewers to decline reviews

Reviewers will receive an invitation to review via email (read about sending invitations to reviewers). It is only when they accept this invitation that they begin the reviewing process (they can also decline to review completely). 

When they login they can see each submission assigned to them and decide for each one whether to Review or Do not Review:

If you wish, you can disable this option for the reviewers so they won't be able to decline to review any submission assigned to them. This can be amended in Step 2 of the  Configure Review menu. 

You can choose to Hide or Show the Decline Assigned Review option as below: