Currently this is only done through the mobile app. However you can disable your account from the web.

• From the home screen tap on the account icon in the top right corner.
• Then tap the settings link and go to Account and click the Delete Account option
• You must validate your account by entering your current password. 

Note:  This will permanently delete your account and all data with it forever.  If you just wish to temporarily deactivate it, select Disable account instead from the Account menu.