Is there a way to show powerpoints on main display but with green screen on alt output?

We use main display for church projector, Alt Output gives us streaming and foldback for the minister and band. This all works well until a guest speaker comes with a powerpoint which displays on all outputs but we want the stream to see the minister speaking and not the powerpoint.

My only solution would be to convert the ppt into images, I know there is a way to do this but don't have PPT any more.

Any ideas would be appreciated. Thanks David