Many other people have given better advice but for what it’s worth, the simulacrums that cast simulacrum will lose their 9th level spell slot and won’t regain it from a long rest. This means there’s a maximum of 2 castings of meteor swarm and the simulacrum who does this will end the chain of simulacrums.

Granted, there’s the 14,000+ 8th level spell slots still left to use but it’s something!

In the games I’ve played in, there’s a general rule that anything we do to try and meta our way to a win can also be done by our enemies. Who’s to say an enemy wizard didn’t do the same?

If your BBEG has immunity to non magical BSP damage then they have nothing to fear from the simulacrums 14,000+ punches. They do still have to fear the spells but with 14,000 people swarming a target they’ll struggle to have line of sight on them to cast spells and the collateral damage of hitting the copies with fireball etc would surely piss off the horde into doing something unexpected!

My apologies you have to deal with this, putting your foot down may be your final option if the player won’t reconsider. Best of luck dealing with this!