You can deactivate or permanently delete your account with just one click. To do so, first login to your account. Go to the Settings tab -> Profile Settings sub tab (see image below). Here you can deactivate your account by toggling the switch on (next to 'Make my account inactive').

To permanently delete your account, just click on the red link "Delete my account". Remember, once you permanently delete your account you will not be able to log back again to your account. You can always create a new account later on if you like.