I tried to run off a gallon of water from my hot water tank for maintenance. But I can't seem to shut off the plastic spigot now. It is turned all the way off.

Unfortunately this is not too unusual - sediment builds up in the tank, and then when you run the water that sediment can interfere with the spigot seal. Best solution is to replace the spigot - it's not difficult but is time consuming because you have to shut the water off to the tank (both hot and cold lines) and drain the entire tank. Some quick "how to" directions are here: http://www.ehow.com/how_8104640_replace-faucet-hot-water-heater.html

Go get a hose end cap, brass one, at HD or Lowe's and screw it on the end. Has an O ring that will seal the spigot.