maybe you are probably tired of using twitter or you have multiple account and you have been thinking of how to deactivate one. then take this steps to deactivate your account on twitter
Sign in to twitter.com on the web
Go to your Account settings and at the bottom of the page you would see deactivate my account link, click on it.
Read the account deactivation information. Click Okay, fine, deactivate account.
Enter your password when prompted and verify that you want to deactivate your account.
After all these steps, you would see a notification screen (page) and you would be automatically logged out.