Venues can be designated as a one time venue. There is a checkbox on the Edit Venue form to configure this. A one time venue is a venue that will only ever be attached to one event. An example of this would be a private residence. One time venues will not appear as an option on the Venue dropdown list on the Add/Edit Event Form unless the event is assigned to that venue. By implementing this approach the Venue dropdown list will be much cleaner, free from venues that you won't use again.

You can choose to give a Venue a One Time Venue value on the Settings Page while adding a new Venue.