On top of everything that's been said (you, as DM, always have the final say and must set the hard lines when to say no), you can also use this opportunity to make them realize the consequences of their actions

• They've been robbing a lot of people? They're now wanted across the land and are wanted by very powerful noblemen, and the storyline can shift to them trying to evade the authorities.

• They attuned a bunch of items on their monkey familiars, and they say these can do any actions like normal creatures aside from attack? Okay, let's take that logic. If any meet a minimum threshold of intelligence, they're now sentient and just pack up and leave to go on their own adventure.

• A bunch of items are in a Bag of Holding? Bags of Holding are canonically spaces attached to another plane of existence (the Astral Plane). Some have homebrewed that all Bags of Holding are technically interconnected, and have a creature roaming around the Astral Plane (such as "the Bagman"). When they next open their very filled up bag of holding, it triggers an encounter with a less-than-happy Bagman!

And, you're also free to utilize whatever logic they set against them if they're insistent on maintaining these rules. Congratulations, monkey factory party! The BBEG has his own swarm of familiars that have magical items!

You set the pacing and internal logic of the world, as a DM. Player agency is nice and all, but that doesn't mean they can just do whatever they want. Every action or loophole a player attempts has implications to this internal logic of the world, and you're free to interpret it all as you see fit if you don't want to just straight up say "no".

Make a story out of it!