stwittergy products offer a free 30-day trial on new accounts.
If you cancel your paid account before you are billed 30 days after your original signup then you will not be charged.
Once you cancel you will not be charged again, but you are responsible for whatever charges have already been incurred for the current billing period. For example, if your billing cycle is on the 18th of every month, and you cancel on the 24th, you will still have to pay for the current month, but you will not be charged again after that. In order to treat everyone equally, no exceptions will be made.
You can cancel your account at any time by logging into your account and selecting the 'My Account' link (usually found on the top navigation bar). Once you are on the My Account page, click the 'Please cancel my account' link.