Create a custom role that can add but not remove reservation items
You can make a StageStock role that lets someone add items to a reservation but not remove them. You'll need the Manage User Roles permission.
- Go to Administration > Site Administration > User Roles.
- Click New role from scratch. (Or open the closest standard role and click Duplicate, since standard roles can't be edited.)
- Name it something you'll recognize, like "Reservation add, not remove". Under Applies to, choose StageStock, then click Create and choose permissions.
- Turn on Add Items to the Reservation and leave Remove Items from Reservation off.
- Click Save Changes.
Then give the role to the person: open their profile under User Administration, go to Category Roles and choose the new role for each category. They'll also need a site role that includes Create/Edit a Reservation, which is separate from the per-category permissions.