The chasm will pretty much suck up all the magma you pour into it without never filling up so you can't directly "flood" it. You can however flood the tiles surrounding the chasm (if it's a circular chasm) to kill the monsters that way, but it requires quite a bit of dwarven engineering.Also monsters don't pop out from chasms indefinitely anymore so you could just kill them by amassing a horde of wrestlers there or wait them to die of old age.
[ May 18, 2008: Message edited by: Silu ]