No you will not be charged. You just won't get back any credit of the money you've already paid.
If you don't need the phone number, just log into your account and Unscrbibe from the Manage Subscription tab. After 90 days of having no payment or Available Funds, the automatically be closed.
If you want to keep your same phone number, do not cancel the Public Mobile account. You must find a new provider and request to port your phone number to them. Once the port has been completed successfully, your Public Mobile account will be closed.
