Hi Paul,
You can edit the data through the map viewer.
Log into Survey123 (http://survey123.arcgis.com/) > Open the survey in question > Data > Click 'Open in Map Viewer'> Edit > Click on a point on the map you want to delete and the Delete button will be in the pop up.
If this doesn't work for you, log into www.arcgis.com > Content > find the data and add it to a new map > Edit > Click on a point on the map you want to delete and the Delete button will be in the pop up.
Hope that helps!
Carmel