Example migrated from Codex:

The following example removes an action from the Bulk Actions drop-down on the Users page:

<?php
    function my_custom_bulk_actions($actions) {
        unset( $actions['delete'] );
        return $actions;
    }
    add_filter('bulk_actions-users','my_custom_bulk_actions');
?>

Pay attention to the actual option values, not just the text displayed in the dropdown, on the page you wish to modify. For example, the delete option on the Plugins list page is “delete-selected” (as of 4.2.2).