You haven't supplied enough information about your use care for any reasonable person to help you.
Can you explain more about why you want to block accessing other belt items? The whole point of the belt items is for quick access. How do you intend for players to be able to use the other belt items when they are blocked? If you don't want to allow using other belt items at all, then you could use the approach of simply disallowing items being placed in those slots. If that doesn't work for your use case, then there are clearly other details about the user experience that you have left out.
I succeeded in locking it, but
Slot 1 is unlocked, but the item disappears.
If you turn it back, the item is still there.
player.inventory.containerBelt.capacity = 1;
player.inventory.SendUpdatedInventory(PlayerInventory.Type.Belt, player.inventory.containerBelt, true);
I'm making a weapon damage upgrade.