The sorcerer's Subtle Spell metamagic option states:
When you cast a spell, you can spend 1 sorcery point to cast it without any somatic or verbal components.
The description of material components (which subtle spell does not remove) states in part:
A spellcaster must have a hand free to access these components, but it can be the same hand that he or she uses to perform somatic components.
From these two, I'd conclude that a bound and gagged sorcerer could not cast any spell requiring material components. (For safety, we might also want to remove the sorcerer's focus or components bag if possible.)
However for spells without material components, is there any way to prevent a sorcerer from casting, short of conditions that deny him actions (incapacitated, petrified, stunned, or unconscious)? Charmed would prevent hostile spells, but not necessarily escape-type spells.
Suppose you had a captured and unconscious sorcerer and wanted to wake and question him, but believed he might have access to Subtle Spell — is there any safe way to do this?
The party is 5th level, and we have a monk, a war domain cleric, a sorcerer, a barbarian/druid, and homebrew variant barbarian 1/life cleric 1/wizard(abj) 3. We have a couple of NPC allies as well. There are a bunch of dead knights around, so grabbing additional weapons and armor is possible. No magic items except healing potions.
Best Answer
Put him in armor he's not proficient in.
In 5e you cannot cast spells while wearing armor you're not proficient in. So unless the sorcerer has dipped Fighter and is therefore proficient in all armor (one of the quirks of 5e), wearing armor he's not proficient in renders him unable to cast spells. Plus, the armor makes it more difficult for him to escape without the time to remove it. Make sure to keep him tied up at all times though. So when adventuring it pays to bring along some extra armor, just in case you encounter evil enemy spellcasters (or evil Druids in earlier editions, where Druids became unable to cast spells or Wild Shape when wearing metal armor).
Note that this will work only once, and a savvy DM will have his villains minor in Fighter while attening Evil Spellcaster School to get a workaround for this.