If you for some reason don't want to keep your Karma account, go to Settings > Edit Profile to find the option to delete your account. For security reasons, you are asked to enter your password in order to delete your account.
For iPhone users: If you have created your account with your phone number, you will first need to reset your password. To do this, log out, select the Email login option, and then Forgot password. Enter your email address and an email will be sent to you with information on how to reset your password!