there are not any standard rules for disenchanting magic items, with a few exceptions, as follows:
- the ever famous "mdj" or mordenkainen's disjunction can disenchant magic items. as a side note, an epic spell could easily duplicate this effect.
- the artificer class from the eberron campaign setting book has a class feature that has this effect. sixth level, if i recall off the top of my head.
- not explicitly listed as a valid effect, but wish and miracle effects and the like are able to create items with certain limits, so it stands to reason that un-creating similar items should easily be within the realm of possibility.
- crude, but technically legit, simply breaking the item will relieve it of its magical properties. and when repairing, don't re-enchant it. (note-not recommended to be attempted with a staff of the magi, or other similarly enchanted items.)
- some charged items can be drained, by either usage, or by certain other magic items which render them normal items again.
- and last, but certainly not least, if the dm says so, then it is. ^^ but the above could certainly be used as guidelines. houserule to the rescue!