[Skarg]

If you have a wizard cast a strong Curse on them, that'll tend to make things hard for them.

If you have a wizard with the Geas spell, you can have others cast Confusion and/or Curse on him (to reduce the prisoner's resistance), then Geas him not to cast spells until released.

The Little Death will also do it, at least if the prisoner has some ST left and you only need them to last a few days.

It's also not much of a stretch to invent a variation of Pentagram which would work to block spells coming from inside it, for a cell for a wizard prisoner, possibly even using it on a barred wagon.