I would blindfold the psion so he can not select any guards as targets. Better yet put some iron mask on him with a lock so it does not fall off by accident. :)
ThisI had a character once, a Mystic Theurge, for whom nearly her entire spell list (particularly prior to her mid teen levels) was chosen with the knowledge that a large chunk of it would be used with Still, Silent, or Eschew metamagic applied. One of her favorite ways to enter combat was in Gaseous Form which didn't slow her casting ability much at all. Along with emphasis on "face" skills such as Diplomacy, Bluff and Sense Motive she was a walking nightmare to imprison in anything resembling a normal manner. Of course much of her dangerous nature was because 99+% of the time she wouldn't appear any more dangerous than the next 'joe' so simply knowing who/what you are dealing with is most of the battle.
1) Keep all portions of the cell interior visible to any guard(s) looking in. No corners, no space 'behind' the door, no bunk with sheet to hide under etc..
2) Block LOS ... blind folds, a helm with solid visor, or even just a cloth bag over the head are all very useful and very mundane ways to prevent anything/spells/SLA's being used which require targeting whether an object, creature or point of origin.
3) Do what law enforcement everywhere does, outnumber them!. Got one dangerous prisoner to move/handle do not bring just one, or two, or three guards ... bring 6 or 8 and be prepared to call in backup as needed asap. Guards at range with a bars, walls and windows/arrow slits between should be present as well to freely use attacks at range while protected from melee by said bars, walls and windows should the prisoner attempt anything.
4) Use your "Special Substances and Items": Tanglefoot and Thunderstones etc.. Guy moves 'wrong' and they should be covered in goo at the least.
5) The prisoner should be blind with shackles and manacles every time they leave the cell (or even all the time) and have the prisoner stick his hands/legs/limbs out to place as many restraints as possible on him before the cell door is even unlocked or opened.
6) Unless necessary don't directly handle/touch the prisoner. Use a mancatcher or rabies pole (solid staff with noose) or similar devices, potentially in pairs, so the guards can pull in opposite directions as needed for control. Nothing says the prisoner can't be told to move directly from the cell into a wheeled barred cage to be rolled where ever for questioning, court hearing etc..
7) Drug them into a stupor ... exact game mechanics up to you.