EVE Online account subscriptions can be cancelled at any time by selecting "Cancel Subscription" from the Account Info page.
Start by logging into Account Management and click your account name in the top right hand corner of the page and select "Account Info":
From there, look at the center section for the Subscription and payment window:
This option is only available if the account is currently on an active and recurring payment plan (Credit Card or Paypal payments).
Cancelling the subscription will revert the account to alpha-state once the remaining game time runs out.
Clicking “Undo Cancel Subscription” after the Cancellation confirmation will place the account back onto the previous recurring payment cycle, resulting in a continuation of the subscription and further payments as if the subscription had not been cancelled at all.