You can deactivate your Twitter account on iPhone with very simple steps.
First, go to the Twitter.com and login to you account
Click on the “Account Settings” link at the bottom of the page, click “Deactivate My Account”, click “Okay, Fine, Deactivate My Account” button to confirm your account deactivation.
Your user data will be permanently deleted around 30 days from the date of deactivation. And you can reactivate your account in this time by logging in.