WordPress - Example remove action registered with $this object reference.

example.php

<?php

class Foo {

  public function __construct() {
    add_action( 'init', [ $this, '_bar' ] );
  }

  public function _bar() {
    exit( "Request Blocked, action was not removed" );
  }


}

new Foo(); // or $bar = new Foo();
// essentially, $bar would not be a variable accessible by the code below, because it was defined in a totally different namespace or environement.


global $wp_filter;

if( isset( $wp_filter[ 'init' ] ) ) {
  $callbacks = $wp_filter[ 'init' ]->callbacks;

  // We assume that the priority will always be 10, in case the priority is not static for whatever reason, we need to loop trough all the priorities.

  if( isset( $callbacks[ 10 ] ) ) {

    foreach( $callbacks[ 10 ] as $callback_index => $callback ) {
      // Normally this would not happen, but lets assume we need to do integrity checks.
      if( !isset( $callback['function' ] ) )
        continue;

      // If it's not an array, it's a function or closure, and in this example we don't take closures in the digging, but essentially those are even nastier.
      if( !is_array( $callback['function' ] ) )
        continue;

      if( !( $callback['function' ][ 0 ] instanceof Foo ) )
        continue;

      // In this case, we remove everything, but you would target specifically for $callback['function' ][ 1 ] by simple checking if $callback['function' ][ 1 ] === '_bar'
      remove_action( 'init', [ $callback['function' ][ 0 ], $callback['function' ][ 1 ] ] );
    }

  }

}