You can make occupier bookings placed via the User Portal tentative until they are confirmed by an admin. You can easily enable this feature via Settings > Bookings and calendar settings.
Enabling one or both toggles will require an admin to approve every booking made by occupiers with or without user group subscriptions when they book via the User Portal.
Tentative bookings have a grey tab to help you spot them more easily on your calendar.
All you have to do to approve the booking is click on it and disable the This booking is not yet confirmed / tentative toggle.
Once a booking is approved, occupiers can edit the booking.
Any changes made to the booking don't require admin confirmation past the initial confirmation.