Edit caps: Only site admins can disassociate group docs.

only-site-admins-can-disassociate-group-docs.php

<?php 
add_filter( 'bp_docs_map_meta_caps', function( $caps, $cap, $user_id, $args ) {
	if ( 'bp_docs_dissociate_from_group' === $cap ) {
		if ( user_can( $user_id, 'bp_moderate' ) ) {
			$caps = array( 'exist' );
		} else {
			$caps = array( 'do_not_allow' );
		}
	}
	return $caps;
}, 20, 4 );