How do I stop people from ordering more than the available pre-order product inventory?

To prevent shoppers from adding more than the available pre-order inventory to cart:

Go to the Default Settings tab
Check the box Prevent Customers from Ordering More than the Preorder Stock Available in the Stock Warning section.
Click save.

IMPORTANT:

This feature applies only to pre-order settings using Inventory Management Setting #1 or the Specific Preorder Stock feature
This feature only stops shoppers on product pages from adding more than the available pre-order inventory to the cart. It cannot prevent shoppers on the cart page from increasing the item quantity.