Make sure the presentation is saved (possibly as a copy if you want a version with notes).
File > Info
In Prepare for sharing > Check for Issues > Inspect document
Tick Notes and click INSPECT
When given the option choose 'Remove'
source: https://answers.microsoft.com/en-us/msoffice/forum/all/remove-all-notes-at-once-in-power-point/8dbeb41d-0e3e-47de-bd84-5782b71c57aa
