You won't find any automated way to delete a Roblox account on its website. Use one of the 3 methods listed in the post to delete Roblox.

How to cancel your Roblox premium membership If you have an active Roblox membership at this time, make sure that you cancel it before leaving your account idle for a year.

Roblox Premium offers new ways to trade and discounts, but costs at least several dollars a month: If you're tired of paying, here's the best way to cancel.