Yes. After you've supported the server with what ever rank(iron, gold, diamond) you can cancel the subscription right away under the same menu "Upgrades" as from where you will "donate". This will give you 1 month of benefits and supporting status.
To reiterate what everyone else has said: Just act like you'll be paying automatically every month forever, but then click "Cancel Subscription" at the top of the upgrades page. That won't take away any benefits until the month is up. Really what it does is just ensure that you are only a supporter for the rest of the month in which you cancel you subscription.
