Archive activity types from the list in Play → Activities → Types: tick the types and choose Action → Archive. Archiving doesn't cancel sessions that are already scheduled.
How to archive
Go to Play → Activities → Types.
Tick the types you want to archive.
Click Action.
Choose Archive.
How to reactivate
Go to Play → Activities → Types.
Click the Active filter and choose Archived.
Tick the types you want to reactivate.
Click Action.
Choose Reactivate.
Good to know
Archived types no longer show when players browse activities in the app. Sessions that are already scheduled aren't cancelled and can still be booked.
Cancel any upcoming sessions you no longer want separately. See "How do I cancel several upcoming activity sessions at once?".
To hide a type from players without archiving it, turn on Hide activity type in mobile app and widget in its settings.