At this time there is not a way to get Robux back once you have made a purchase in the Catalog. Deleting an item will not return the currency to you.
We strongly recommend you take advantage of the various Roblox games that allow you to test items. You can use these to determine if you like an item before making a purchase. You can locate one of these test places by typing the word Catalog into the search-bar at the top of the site and choosing to search in games.
As a helpful feature to prevent accidental purchases, Roblox includes a pop-up window, asking you to confirm your selection before completing a transaction. If, however, you are having an issue with a Roblox item that you have purchased, please contact Customer Service. Be sure to provide your username, item name, link to the item, and a detailed explanation of the issue.