In the text of spells and magical items in DnD, the word “incantation” is often used.  This is a fancy way of referring to the verbal component of a spell.  According to the Player’s Handbook, verbal components are:

Most spells require the chanting of mystic words. The words themselves aren’t the source of the spell’s power; rather, the particular combination of sounds, with specific pitch and resonance, sets the threads of magic in motion. Thus, a character who is gagged or in an area of silence, such as one created by the silence spell, can’t cast a spell with a verbal component.

Player’s Handbook, 5e

Spells may or may not have Somatic and Material components required to cast them as well.  Prevent a caster from being able to fulfill any of the component requirements of a spell make it impossible for the caster to use the spell.   The text above lays out two ways of preventing a spellcaster from being able to fulfill the verbal components but here are others.

No Incantations Please!

It may seem a little obvious to say, but any spell that imposes unconscious, paralyzed, stunned or incapacitated conditions prevents a creature from taking any actions, and so also prevents spellcasting.  One can assume that these also prevent the creature from speaking, so spells with a verbal component are clearly out. 

Silence isn’t the only spell either.  Dark star creates a zone in which no sound can be created, which would also block the use of verbal components.  The symbol spell inflicts a number of conditions, including including insanity and discord.  Both effects would prevent the use of verbal components.  I feel like there’s a way to get the job done with a 9^th level modify memory spell, but I haven’t quite figured it out. Once I had a player try to prestidigitate an egg into a caster’s throat.  I feel like that would do the trick, maybe.

But if you need a more physical approach, the gag above might be your speed.  There are a few other physical ways to block verbal components.  If you can get the caster underwater, they’re faced with a nasty decision.  According to the rules, the can cast, but begin to suffocate immediately thereafter. Grappling?

Spellcasting Without Verbal components

There are two ways to get cast spell that have verbal components without the verbal component.  The first is the subtle magic metamagic option that sorcerer have.  This metamagic allows the caster to skip the verbal and somatic components of a spell. Innate spellcasting can also allow for a spell to be cast without its verbal component.  This is fairly rare, though, and most instances of innate spellcasting just remove the need for material components.