You gotta have your game plan for why the curse isn’t solvable
Maybe it’s a disease, not a curse. Maybe it’s an evil magical sword, but it’s not cursed per se, it’s just an asshole. Maybe the magic that operates the McGuffin is set to detonate like a bomb if you make an attempt to disable it. You can feel the balance of energies, such that a slight slip will release an incredible conflagration. Do you really want to continue? You’ll have to do a wisdom check with a pretty high DC. If you’re down for that though, you might be able to remove the curse.
Yeah I know that’s not what it says in the books. This amulet isn’t from the books. Do you want to keep going with remove curse? Or try to find another way?
There are always solutions. Remove Curse is bullshit; IDK what they were thinking with it. But yeah you just gotta plan ahead a little bit. Let your players still have their agency; don’t just declare that it doesn’t work (unless you all wanna agree to house rule that it just doesn’t exist or something). Just plan your way around it. Best case is something like your players looking for some way to buff their wisdom enough so they feel confident taking on the Remove Curse, and then you all get to find out what transpires.
Yeah I know that’s not what it says in the books. This amulet isn’t from the books. Do you want to keep going with remove curse?
But that's ALSO stupid. Much better to simply not have the spell. As a player, it feels much more shitty to have a DM "cheat" around an ability that to never have an ability
Let your players still have their agency; don’t just declare that it doesn’t work (unless you all wanna agree to house rule that it just doesn’t exist or something).
Like I said, I agree with you about not just cancelling it entirely unless there's an OOC discussion about making a house rule about it.
You can't let the players ruin your fun (and, likewise, their own, because it turns a challenging situation into some stupid anticlimax and basically removes curses from the game as a mechanic). Likewise you can't ruin your players' fun and just unilaterally say this curse is removal-proof. Fortunately, there are multiple middle ground third options available, that preserve the fun for all parties involved.