We will sometimes buy lemons for other cleaning around the house, but we don't use the whole thing. We used to keep the rest of it around until the next major usage of the garbage disposal, but they would go bad.
To reduce the wast, what we did was slice up the leftover lemon and put it into a container in the freezer. Whenever we start smelling the disposal, we pull out a slice of frozen lemon and throw it in to the disposal. It takes care of both the ice and citrus suggestions while reducing our waste of lemons.
(In case you didn't know, Tom.com is my hubby. :)