buddypress.php

<?php

add_filter( 'bp_get_template_part', 'sippis_short_circuit_activity_post_form', 10, 3 );
function sippis_short_circuit_activity_post_form( $templates, $slug, $name ) {
  if ( 'activity/post-form' === $slug ) {
    return false;
  }

  return $templates;
} // end sippis_short_circuit_activity_post_form