We are truly sorry to hear that you no longer want to be on UW :-( Feel free to contact us with feedback, we are always looking for ways to improve.
You can only remove your account via the website (this isn't possible though the app).
From the website, go to Menu > Settings. At the bottom of the page, you will find a button that you can use the remove your account.
When you delete the app from your smartphone, your account will still exist and your items will still be online. They only go offline when you delete your account through the website.