How To Delete All Gmail Messages At Once

You can delete a Gmail easily be selecting it  and clicking on the 'Delete' button at the top of the Gmail homepage. You can also click on the Delete button at the top when the email is open.

Deleting Your Twitter Account

As much as there are several reasons not to delete your twitter account, there are also many different situations that compel you to want to get rid of your Twitter account.
I don't know why you want to delete your Twitter account. I only know why you don't want to delete it. You don't want to delete your account to change your username or email address. You can do that from right within your account.