Solution:1
Inside class
add_action( 'some_action_hook', array( $this, 'some_function' ) );
Outside class,
With use of global vairable:
global $my_class;
remove_action( 'some_action_hook', array( $my_class, 'some_function' ) );
Using class name:
remove_action( 'some_action_hook', array( 'MyClass', 'some_function' ) );Solution:2
To extend on Rikesh answer: Sadly, using class name will not work. As it says on https://codex.wordpress.org/Function_Reference/remove_action
If an action has been added from within a class, for example by a plugin, removing it will require accessing the class variable.
This leaves only:
global $my_class;
remove_action( 'some_action_hook', array( $my_class, 'some_function' ) );
Or in case of a singleton class like Jetpack to remove the ‘show_development_mode_notice’ hook (for example) like this:
remove_action( 'jetpack_notices', array( Jetpack::init(), 'show_development_mode_notice' ) );
Inside class