A disenchanter is an annoying monster found in the deeper levels of the dungeons and in Gehennom. The random R at the Castle may be a disenchanter, but they are otherwise only randomly generated in Gehennom. Its attack can decrease the enchantment of weapons and armor (as well as doing an unthreatening amount of damage), and reduce charges on items you hit it with.
Hitting a disenchanter with a weapon will decrease the enchantment of the weapon, unless it is an artifact, in which case it has a 90% chance of resisting. Hitting a disenchanter with any chargeable object will decrease its charges—the game treats charges and enchantment the same. Wands are therefore not a good alternate weapon to use to avoid disenchanting your main one.
Disenchanters are the deeper-level equivalent of rust monsters. Disenchanters remove only positive enchantment or charges; a disenchanter will not remove negative enchantment or cause an unenchanted item to gain negative enchantment.
The disenchanting claw attack is subject to magic cancellation, so make sure your armor grants magic cancellation of 3. This does not prevent the passive disenchanting attack. If you zap a disenchanter with a wand or spell of cancellation, its claw and passive attacks will both be unable to disenchant.
Using ranged attacks such as spells or wands can keep your gear safe. If you use missiles, they will be disenchanted, but this is moot if you are shooting rocks from a sling. In a game where you had many scrolls of enchant weapon, you might choose to throw from your pile of +7 daggers; for though the resulting +6 daggers will not stack, they are much preferable to disenchanting your artifacts or armor.
You can keep your weapon safe by punching the monster to death with bare hands. Any gloves will be disenchanted if you hit with gloved hands; likewise, your boots will be disenchanted if you kick it. You could also use an unenchanted weapon; pick-axes work well for this, as do some classes' starting secondary weapons. Be careful when you use a unicorn horn—it is two-handed, and spellcasting monsters (like liches or golden nagas) may curse it and cause you to lose the use of your hands. If this happens in Gehennom or Moloch's Sanctum can be lethal.
If you have a strong pet that does not use weapons, it can dispatch disenchanters easily.
Disenchanters are a common target for single-species genocide, as rust monsters are not much of a threat once the player reaches Gehennom.
Disenchanters can be useful to classes who cannot reliably cast drain life, as they can be used to selectively reduce the enchantment on a weapon or a piece of armor so it can be re-enchanted to a higher level. To reduce the enchantment on a piece of armor, wield it and strike the disenchanter with it once. Just be careful if you try this with your cloak—you should wear a substitute for MC3 in order to nullify its draining active attack.
In SLASH'EM, disenchanter corpses are safe to eat. Additionally, they are no longer the only monster that can disenchant weapons and armor with the addition of the steel golem, which has also a passive disenchanting attack.
In UnNetHack, eating a disenchanter corpse has a 50% chance of removing a random intrinsic, in a way similar to being hit by a gremlin at night. While potentially dangerous, this could also potentially be used to get rid of harmful intrinsics gained accidentally.
Ask not, what your magic can do to it. Ask what it can do to your magic.