Once you add a personalization to a product (like initials or a monogram) and add the item to your registry, your guest cannot change the text.
View when adding to your registry - when you can customize your personalization choice:
View when guest is shopping for the item on your registry - there is no option to change the customization here.
They will only be able to purchase the item with the selections you made (no changes!) Also, if you choose not to add personalization to an item and add it to your registry, the guess cannot add any personalization on to the product.