How to remove/hide the predictive autocomplete popup when I type in the tag/term metabox

There is no filter or way to stop those Ajax calls. However, you can alter the response of Ajax call by using filter term_search_min_chars.

Which is actually to customize minimum number of characters after that Ajax will print the response. So we can make it 0 to never print the response!

/**
 * Remove tag search suggestions
 * @param int $tax The minimum number of characters required. Default 2.
 * @return int
 */
function remove_tag_suggestions($tax) {
    //You can place your condition if you want!
    return 0;
}
add_filter('term_search_min_chars', 'remove_tag_suggestions');

PS: I am not sure why do you want to remove this wonderful feature!