Since you are not specifying a priority to your actions, they will be executed in the order they are encountered. So your remove_action may very well be executed after the actions tied to the same hook have already been executed. You could solve this by attaching a higher priority to the action that does the removal like this:
add_action('stachethemes_ec_add_event_tab_content','custom_stachethemes_tab_content',20,0);
Beware also that remove_action does not remove the hook itself. It just removes the specified actions bound to that hook. If another action is tied to that hook later on, it will be executed.