I'm curious as to why you would believe that there is a constant flow of energy into them that must be dispelled. You said it yourself, first in the question and again in the explanation. "Magic is a source of energy that wizards have access to." "Magic is a source of power that magic users can draw upon."
It is my belief that magic is not a "source of power" so much as energy is a source of magical power. Everything in the universe has inherent energy. Matter, electromagnetic waves, radiation, everything around you is constantly oscillating.
Matter is held together by energy. Matter itself has a wavelength, albeit a very low one. When a magic user or wizard attempts to cast a spell, I believe he is making use of the energy that is around him, and transmuting it into useful abilities. This is why many people (for example, Wiccans) who make use of spells and magical powers often have many stones and crystals around them. Different materials have different energy levels, different frequencies, and are all useful for different things. Many materials are good for storing energy, so this can be a way for the wizard to gather energy and carry it with him. I do not believe there is any need to "bleed off" unused energy, I don't think it is even right to call it unused energy, but rather it is energy that has not yet been used. Energy that is acting in the way necessary to hold the universe together, until the witch or wizard wishes to make use of it.