To cancel (discontinue) DSL service, please use the NCF Office Message system and select 'DSL Cancellation'. Alternatively, contact the NCF office by phone.
To keep things simple, DSL service is provided on a monthly basis. There are no refunds for a partial month of service. So when you cancel, service continues until the end of your current month block of service. To find out what day your DSL service ends, look at 'monthly extension date' on the DSL home page.
To ensure the billing system doesn't bill you for another month of service, be sure to contact NCF a week or two ahead of your monthly extension date to say that you'd like to cancel. You will be contacted to confirm. If you cannot be contacted prior to your monthly extension date, the billing system will likely bill you for another month -- so be sure to contact NCF with plenty of lead time.
If you know you'll be resuming service at a later date, be sure to mention that when talking with the NCF office.
There is no long-term contract, no penalty fees, no hassles returning a modem (it's yours, you own it), and no high-pressure sales pitch.
(Regarding your modem, if you'd like to donate it or sell it second-hand, NCF has a DSL Equipment Buy/Sell discussion group that might interest you.)