Welcome! This forum has a treasure trove of great info – Scouters helping Scouters! Just a heads up, though - all content, information, and opinions shared on this forum are those of the author, not the BSA.
From the scout’s page, scroll down and click on their membership. Then, click on the current membership in your unit, uncheck the position approved box, and click update. This won’t automatically remove them from existing events, but it will keep them from being invited to newly created events.
To easily update all events, I suggest using the feature assistant extension.