I'm running a mixed level game (2nd & 3rd), and am just about to capture the party to sell into slavery.
How do you know that? Are you just deciding what happens to them? Taking the party prisoner is something you do to prevent a TPK, not something you do because you want to as a DM.
But how do I insure that the Cleric & Sorcerer, don't cast any spells, short of cutting out their tongues?
You just gave yourself the answer. If the slavers are aware that they are dealing in spellcasters, they'll probably just cut their tongues out, blindfold them, and make use of a kangu to prevent useful movement. In many cases though, spellcasters probably won't be considered worth the trouble, and instead they'll be just killed outright. This is particularly true of the cleric, who can't be reformed or bargained with, and whose social status as a priest should be protecting this from happening in the first place.
You can make this less unfun than its going to be in several ways...
a) Do the blindfold and the kangu, but not the tongue ripping. Still stops spells because the caster has no line of sight and can't use somatic components.
b) If the party doesn't use spells, don't have the slavers realize what they are dealing with.
c) Have the slavers force the cleric to violate his beliefs in some fashion, cannibalism, unintentional murder, etc. resulting in temporary loss of spell casting capability until a means of atonement is found. Not sure that its strictly RAW but it might can keep the cleric alive when otherwise there is no logic to it.
d) Some of this depends on setting convention. The problem is you are trying to enslave modern persons possibly using conventions of antiquity - were slaves might ultimately be treated well, rise to high station, buy their own freedom, etc. Try to express through the conversations of the slavers that they think the spellcasters are potentially really valuable, and have the slavers be trying to convey to the spellcasters that if they don't resist, don't try to cast spells on their men, and so forth they are likely to be treated very well by whoever buys them. Maybe even have the slaver apologize to the players saying he regrets they've had to deal with them so harshly. Play against expectations.
There are a variety of other heavy handed techniques, but they have the same flaws as the tongue ripping issue. They also would suggest levels of competence by their captors that are rather extreme, leading what should be heroes into a situations where they are still apprentices.
On the whole though, this doesn't sound to me like its much fun for the players and its really difficult to arrange to happen. Most PC's will just fight to the death, and capturing PC's isn't easy. You've also probably moved in to areas where the social contract needs to be questioned. A lot of realistic 'you've been captured' behavior by enemies results in things that could violate consensual RP. If you are forcing that as a DM, you are engaging in questionable behavior and possible misuse of your authority.
It will be very important to if you don't end the slavery status soon, to at least end the constraints on personal freedom soon - either by providing for PC escape, or having them purchased by a sympathetic figure who sees them as potentially valued servants and members of his household.