If you want to disable an specific one that isn't custom do this. In django 1.6.6 I had to extend get_actions plus define has_delete_permission. The has_delete_permission solution does not get rid of the action from the dropdown for me:
Not including it in actions = ['your_custom_action'], only works for the custom actions (defs) you have defined for that model. The solution AdminSite.disable_action('delete_selected'), disables it for all models, so you would have to explicitly include them later per each modelAdmin