There are several spells which ward against teleportation, planar travel, or divination spells. But as far as I've been able to find, there isn't anything that would stop the Sending spell from penetrating an area. Is there any magic, item, or other method that can achieve this?

Use case: A Wizard traps an NPC in a demiplane, using it as a prison. The Wizard knows the NPC's allies could cast Demiplane to get him out, if they knew the exact contents of the demiplane. The NPC can tell them exactly what is in the demiplane if they contacted him via Sending. How can the Wizard stop the NPC from escaping (by blocking the Sending spell)?

Related: How can you gain selective protection from Sending?, but here the protection needs to be (a) non-selective and (b) involuntary.

An antimagic field will do the trick.

The 8th-level spell Antimagic Field states:

Targeted Effects. Spells and other magical effects, such as magic missile and charm person, that target a creature or an object in the sphere have no effect on that target.

Notably, however, Antimagic Field is not a permanent solution, as it requires your concentration, is centered on you, and only lasts an hour.

Wish for it.

This is always an option, but probably not what you're looking for. Still, for completeness, I felt like it should be included. Note that it's up to the DM to specify whether blocking an area from Sending is within the power of Wish, but as spells like Forbiddance can block planar travel, I'm pretty sure that shouldn't be an issue.

Alternative solutions to your use case

The following options don't technically ward an area against Sending, but in the use case you gave, they should still fix your problem.

Geas the NPC to not cooperate/communicate with his friends.

This is not foolproof, as the NPC can opt to take damage in order to violate the terms dictated by the spell, but if he doesn't have that many hit points, that may not be an option. In that case, Geas can be cast on the NPC to compel him to not cooperate or communicate with his friends.

If the NPC ignores this command, he/she takes 5d10 psychic damage, which - as mentioned - reduces the viability of this spell on higher-level NPCs. Of course, your Wizard could knock the target out by bringing him to 0 HP, then healing him just a little - that way, the NPC can't risk breaking the Geas, even he would survive the damage on full HP.

Block the Demiplane from extraplanar travel (maybe).

You can ward your demiplane against extraplanar travel with spells such as Forbiddance (Hallow only works once you're inside the spell's area, you can still get in before you're affected). It's not clear whether or not such a spell cast on a Demiplane would prevent opening a portal to that Demiplane with the Demiplane spell, but I'd tend towards yes. Note, however, that you would have to stay on the demiplane, as otherwise, you obviously couldn't return yourself.

How to be certain to block Sending ?

Basically, the "nuke" in term of blocking magical effect is an Antimagic Field. Any magical effect in a 10ft radius, for an hour (or while the caster concentration last), is repressed. There is not a lot of way around this.

Since the spell is Evocation and not Divination, spells and effects like Non-detection would be useless.

How to counter an external Sending to allow a rescue in a prison demiplane

In your use case, there is multiple thing to take into account.

First, Sending :

You send a short message of twenty-five words or less to a creature with which you are familiar. The creature hears the message in its mind, recognizes you as the sender if it knows you, and can answer in a like manner immediately. The spell enables creatures with Intelligence scores of at least 1 to understand the meaning of your message.

Then, Demiplane

if you know the nature and contents of a demiplane created by a casting of this spell by another creature, you can have the shadowy door connect to its demiplane instead.

Here, you don't need to block Sending to make it useless, you just need to hinder the prisoner ability to know and/or communicate the nature and content of the demiplane. There is multiple ways to do so :

Blocking the creature senses

A blindfold and manacles linked to a wall (or more permanent blindness if you're not a kind soul) can be enough : the creature won't be able to explain the environment if they can't perceive it.

Blocking the creature ability to communicate efficiently.

Feeblemind states :

The creature can't cast spells, activate magic items, understand language, or communicate in any intelligible way

While Sending never talk about a "language" and states that creature with an Intelligence of 1 still understand your message for the "receiving" part, I would say that the "can't [...] communicate in any intelligible way" of Feeblemind counter the reply. Even if it doesn't, the creature have now the mental ability of a Gelatinous Cube to explain the demiplane nature so their response would be... interesting.

Change the prisoner make them an invalid target for Sending.

Sending's target must be "a creature with which you're familiar". True Polymorph the prisoner into a chair : they're not a creature anymore, Sending won't work on them. Just Dispel Magic when you need to interrogate them.