BuddyPress.org

@sakarya simple way: remove the whole form in the /activity/post-form.php file and copy that file over to your BP Default Child theme or WordPress theme in server following the directory structure. Or, remove the call to /activity/post-form.php in three files which you then need to copy over to your theme folder in server
See https://codex.buddypress.org/developer/theme-development/a-quick-look-at-1-7-theme-compatibility/