For Network IPS appliances, you can remove any connection event from the list. However, if you edit a predefined connection event and decide to remove it, the event is not returned to its predefined state. Instead, the event is removed from the list entirely.
If you want to use this event again in the future, it is no longer available. Consider disabling the event and keeping it in the list. This way, if you want to use it again at another time, the event is still available to you in some form.