How to get rid of past events in the Event field lising (Select Box) on New Event Registration page Page
Don't think there is a setting in the UI to do this. The event is basically added to the input widget using the getlist api of Event entity. Adding the following param to the same should be able to list only upcoming events in the input field.

'params' =&gt; array('start_date' =&gt; array('&gt;' =&gt; "now")),


You should be able to do this via an extension. Or if you want to see the param buildup in core, it is present in Event/Form/Participant.php

Extending the above array to include our param should remove the past events.

$eventFieldParams['api']['params']['start_date'] = array('&gt;' =&gt; "now");


Note that changes made to core would be lost after an upgrade, so it is better to override this file in an extension OR see if any hook can help you in attaining this.