Yeah, if you just construct something over the glowing floor before the sword is removed, Hell isn't revealed.
These types of thread are always fun because we can, completely honestly, assure unsuspecting victims that the demonic fortress isn't actually that dangerous to enter, and that getting to the upright sword isn't actually that difficult
What we typically don't mention is that actually taking the sword is probably the most dangerous thing that can be done, for the reasons you've noticed. I suspect that people who have been lucky enough to have avoided spoilers are warned that there's
something up with the forts from all the vague encouragement they get, but they seem to usually guess that it's the fort itself, rather than what's below it, that is being hidden.
Anyway, easy ways to kill demons include: cave-ins, long fields of upright spike traps linked to a lever that is pulled on repeat (trapavoiders don't have immunity to those), and obsidianization. Filling a long hallway with magma, letting the demons into it, and then letting water flow over the top of it (preferably through grates) is an easy, fairly repeatable method of killing the demons, but personally I prefer the spike trap route. Mind you, it takes about 40-50 strikes from steel spikes to kill an organic clown, so even with 10 spikes per trap you'll need a fairly long field, and any fire demons will destroy the trap that kills them and possibly adjacent traps unless they're made entirely from adamantine (hellfire is literally hotter than the sun and fire demons break into a cloud of it when they die).
Edit: Also, I should mention that if the path from the depths to your fort proper is entirely sealed off, eventually your FPS should return to its natural levels as the clowns map the lack of routes. Of course, if/when you open it up again it'll fall, but that will make it easier to build up proper defenses.