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.
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.