Mdtryhht

I'm currently DMing a game where one of the players is playing as a Forge Cleric. As part of the story and due to some of their actions, the party is likely going to be put in jail next session. I'm wondering if there's any way to prevent the Forge Domain cleric from simply using their Artisan's Blessing Channel Divinity option (XGtE, p. 19) to escape.

Normally, I'd prevent the cleric from coming in contact with any metal in jail, but that's not ideal for my campaign/setting. This player is also a stickler for the rules, so any RAW solution would be especially good. Also, this jail is run by wealthy individuals, so expensive/rare solutions are fine.

Antimagic

Channel Divinity is magic:

At 2nd level, you gain the ability to channel divine energy directly from your deity, using that energy to fuel magical effects.

This doesn't have to be the Antimagic Field spell - smart jailers build their jails on natural antimagic areas.

The cleric can't just transform their metal cell door to escape

It seems that your concern is that, should a Forge Domain cleric be imprisoned in a jail cell made of metal, they could use Artisan's Blessing to deconstruct the metal bars by transforming them into some other object, this creating a hole in the wall that they can escape through. However, this is not possible. Part of the requirement for using Artisan's Blessing is (emphasis mine):

As part of this ritual, you must lay out metal, which can include coins, with a value equal to the creation.

The requirement to "lay out" the metal implies that the metal to be transformed using this feature cannot be part of a structure or other larger object. Even if the cell door is made of metal, the cleric is not capable of laying out this metal in the manner required by the ritual. Similarly, it wouldn't make sense that the cleric could disable a trebuchet made mostly of wood by transforming the metal hinge into something else.

If the cleric could get their cell door off its hinges and lay it down on the floor of their cell, then they could probably transform it, but at that point they have already escaped their cell.

The antimagic field spell or (although probably not what you are looking for) the sleep spell. Another spell option is imprisonment.

If you want to make this spell portable, being the DM, you are allowed to create your own magical items (see page 284 of the Dungeon Master's Guide), so you could create something like an orb that allows the jailer to cast the spell, or that creates a constant antimagic field around it or something. As the DM, that is all up to you. Hope this helps.

Petrification

A prisoner who is pertrified certainly won't be making any escape attempts. Additionally, they don't require feeding and cleaning. :-)

Harsh but effective: Cripple them

Cripple prisoners' hands, tongues and eyes (either by injury, binding, or removal). The wealthy owners can afford to pay for something like greater restoration when a prisoner is released or needs to be interrogated.

Less harsh option: Threaten to cripple them

Tell prisoners they are on the honour system. But if even a single one of them steps out of line then they will all be crippled as above.

Concentration

Rituals generally require concentration, so interrupt prisoners irregularly with environmental effects that break concentration.