You will need to permanently reduce their intelligence in order for this to work as if it’s only temporary when the temporary change ends the target will become an invalid target for awaken.
I do not know of any way a PC can permanently alter the intelligence of another creature outside of wish which may come with it’s own consequences for a request like that.
Here are a couple of options.
Both of the below require giving an NPC an immensely powerful item.
Giving an artifact with major detrimental properties to the creature
The DMG has an option for DMs to give artifacts (and other magical items) beneficial and detrimental properties.
Each artifact has its own magical properties, as other magic items do, and the properties are often exceptionally powerful. An artifact might have other properties that are either beneficial or detrimental. You can choose such properties from the tables in this section or determine them randomly. You can also invent new beneficial and detrimental properties. These properties typically change each time an artifact appears in the world.
An artifact can have as many as four minor beneficial properties and two major beneficial properties. It can have as many as four minor detrimental properties and two major detrimental properties.
If you were to gain access to an artifact, and have it exhibit the following major detrimental property, then it could work:
When you become attuned to the artifact, one of your ability scores is reduced by 2 at random. A greater restoration spell restores the ability to normal.
However, you would need get an artifact and have the creature attune to it. And have either the DM provide this property or have it randomized. And you’d have to hope no one cast greater restoration on it after.
Giving a Deck of Many Things to a creature and hoping they pull the right card
Thomas Markov suggests this as another possibility with drawing the idiot card.