My question is how to have customers not to buy products that are out of stock?
I have multiple variants and my inventory quantity is at zero for my large and extra large.
Can someone help me to solve this problem?
Make sure to uncheck the box "Continue selling when out of stock" for each of the variants.
For reference:
You can see “Continue selling when out of stock” in inventory tab of product edit page.
By remove check of this you can prevent customers to buy products out of stock.
Veeery easy. 
Hi @Nick_Marketing and @ashtoncullen ,
I still have customers ordering products out of stock although both Track Quantity and Continue selling when out of stock is uncheck marked.
Your solution doesn’t help in Shopify.
I solved the problem by removing the product from the multiple variants and reenter them in a later date.