demo.php

<?php
/*
Plugin Name: Testing
*/

class Demo {
	function do_a_thing() {
		echo '<!-- demo -->';
	}
}

$instance = new Demo();

add_action( 'wp_head', array( $instance, 'do_a_thing' ) );


// problem: without access to $instance, removing this hook is hard
// solution:

add_action( 'plugins_loaded', 'remove_it' );
function remove_it() {
	global $wp_filter;
	foreach( $wp_filter['wp_head']->callbacks as $priorities ) {
		foreach ( $priorities as $name=>$action ) {
			if (
				isset( $action['function'] )
				&& is_array( $action['function'] )
				&& isset( $action['function'][0] )
				&& is_object( $action['function'][0] )
				&& is_a ( $action['function'][0], 'Demo' )
				&& $action['function'][1] === 'do_a_thing'
			) {
				remove_action( 'wp_head', $name );
			}
		}
	}
}