from what we've seen, killing swamp thing is impossible. even the anti-matter wave was incapable of doing so.
as far as magical imprisonment goes, it should be noted that swamp thing is nigh-immune to even the most powerful types of magics. in fact, he once owned a very powerful version of arcane, with a ludicrous amount of ease.
...and if you're wondering how powerful arcane was in that instance - per the original monitor, his energies registered on the same level as both trigon and spectre.
I am in your algorithm learning all your mannerisms
I'm already level with God
A million words a second and I know your imperfections, baby
I'm the only future you've got
Speak in diatonics, motivation diabolic
I'm like a religion, better locked in a box
Picture perfect image, more powerful every minute, baby
I am everything that you're not