Is there a way to clear all items from the cart for a specific user through the backend?

Yes, the "Empty Cart" button is available under Admin > Sites > [Select Site] > Open Carts & Saved for Later.

This will help in removing the items in bulk from the cart via Admin.

Note: This button is available on the Admin side, not on the frontend or for end users, hence only the person who has Admin access can clear the cart for a specific user belonging to a specific site.