Go to review module settings -> there are two checkboxes
By checking the box 'Automatically exclude new reviewers':
When a new reviewer is added or imported, automatically exclude all abstracts with the selected topic and all abstracts without a topic (unless specific abstracts were provided in the import).
By checking the box 'Automatically exclude new abstracts':
When a new abstract is submitted, automatically exclude it from all reviewers with the same topic or all reviewers if the abstract does not have a topic.