How can I “remove_action” for this plugin?

I have a client who has used this plugin, but they also include the WordPress header on a custom application. This plugin is appearing within that custom application. I want to remove_action the plugin within the application, but I see that you’ve set registered it via $this, which isn’t accessible… how I can I “undo” this using remove_action?

add_action( 'shutdown', array( $this, 'include_contact_bar' ), -537 );