Beholders in T2 are monsters that are generally not worth tangling with - the low XP and poor treasure don't make up for their high HP, annoying spells, and sanity drain attacks.
Omarax is their king (or something like that) and needless to say is particularly nasty. Especially the Summon Aid spell, which can summon other beholders, including the dreaded Hive Mothers; but those mana bolts and nether balls hurt too.
Re "anti-summoning," the term is "anti-summoning corridor." It looks like this:
Code:
....
##.#
#.##
##.#
#.##
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The monster follows you into the corridor, and since the only space adjacent to you and in its line of sight is the one it's occupying, it can't summon anything.
BTW, since you were playing a sorceror... Some vaults make it possible to use some really dirty tricks. If that vault you found was a Greater Cell Vault - just rows of single-square cells separated by granite and permanent walls - then you can teleport tough monsters away when you reach them. They'll land outside the vault, and can't get back in because of the permanent walls (and because ToME's monster AI is terrible). You can then plunder the treasure they were sitting on with impunity (because every cell in a GCV contains a guaranteed excellent item).
NB: this is an extremely cheesy way to tackle a vault, and probably would have been made impossible had DG continued developing T2. As it stands now it would probably be very easy to prevent (e.g. by making uniques resist teleportation).