Short of mage maiming, and assuming no magical abilities of the slavers, I suspect they'd be tied and gagged in transport. They will likely have used some spells resisting capture - preventing them regaining spells seems the likely focus.
For the Sorcerer, simply don't allow him uninterrupted rest. Every " interruption adds 1 hour to the total amount of time he has to rest in order to clear his mind, and he must have at least 1 hour of uninterrupted rest immediately prior to preparing his spells", so make sure he's awakened every couple of hours and worked for over 12 hours a day, or just wake him in the early morning and put him to immediate hard labour so he never has that hour right before spell prep.
The Cleric is tougher, depending on how you interpret " If some event prevents a character from praying at the proper time, she must do so as soon as possible. If the character does not stop to pray for spells at the first opportunity, she must wait until the next day to prepare spells." Is any action taken by the slavers automatic "prevention", or can they disrupt the process? Note that the Cleric needs a full hour in prayer to regain spells. All the guards have to do is prevent that prayer. I would not allow the cleric to quietly pray, still and unnoticed, curled up as if in sleep (assuming the deity is not one of secrecy or some such). Keeping the cleric shackled and gagged when not under direct observation should also prevent regaining spells.
Of course, they would logically also remove divine focuses and spell component pouches, which restricts the spells which could be cast if they were available, at least for the cleric.
When the players argue against the effectiveness of these measures, the phrase "So if you needed to escort a captured spellcaster" may be helpful...