BuddyPress.org

You cannot disable it. The same code is used on profile > activity.
Perhaps you could remove access to the main activity page by redirecting somewhere else.
This will redirect to home page.

function pp_check_if_activity_main_page() {
	
	if ( bp_is_activity_directory() ) {
		
		bp_core_redirect( home_url() );
		
	}		

}
add_action( 'bp_init', 'pp_check_if_activity_main_page' );