The Klevu Search plugin only indexes products with visibility set to Catalog, Search and Search.
If you set the visibility parameter to only Catalog or Not Visible Individually, the product will not be indexed. Please note, the child products with visibility Not Visible Individually, having the configurable products with visibility Catalog, Search and Search are synced to the backend.
Catalog visibility products can also be synced with some workarounds. To sync those products, please contact support.
PS: To exclude certain products, please change their visibility to Not Visible Individually and make sure they are not assigned to any configurable products.