In my story, I have found myself in somewhat of a pickle: Although the hero is more than powerful enough to defeat a villain (which is your typical 'Dark Lord’) I don't think my protagonist is ready to kill a soul just yet (this is not the climax) but there is no prison that can hold said villain, and the hero can't stay there forever (Just a passerby). Is there any alternative to neutralizing a villain that is pretty much pure evil who cannot be contained?
The villain gets hoisted by his own petard and gets killed as a result of their own actions. Spell going wrong, murdered by a traitorous underling, MacGuffin explodes when his plan is foiled, etc.
Alternatively, he gets into a fight with the hero on the roof of a train, with the villain’s back to the train’s direction of motion, and when the hero sees an oncoming obstacle and ducks, the villain doesn’t.
As people are already suggesting, you could seal away or make ineffective your antagonist in a number of way. Sure, you said the enemy can't be imprisoned, but you could find other ways to incapacitate him; it mostly depends on whatever devices your world/plot uses.
For example, you could find a way to shatter his soul in many little pieces, so he would take millenniums to be whole (and dangerous) again.
In Harry Potter, Voldemort does this to himself to become immortal, but the process of dividing his soul in seven pieces actually makes him weaker. Imagine doing it an hundred times ...
Another option would be wiping out his memory completely, making lose his inherently evil personality and reducing him to a less aggressive, less dangerous condition.
In Adventure Time, the Lich - who falls completely in your definition of dark lord, force of nature - gets regenerated into a giant baby with little to no memory of his destructive will. Not so scary now, uh?
A maybe even more questionable course of action would be to exile the evil lord to another plane of dimension (and let those other people deal with him).
Story wise, thought, you have to make certain to do this right. You will have spent considerable time telling your readers than the dark lord is evil; if, in the end, after the final showdown, your hero doens't kill it, some more practically-oriented readers (such as me) will be at least a little disappointed.
There's nothing wrong with your hero not being ready to get his hands dirty, but he must face the consequences of his actions. What are the moral implications of leaving such a being still alive and able to come back? The story must elaborate on this to give the reader a satisfying ending; too many stories end with the hero taking the high "no-kill" ground, where in real life, this would be unpracticall and probably morally wrong too.
Also, how would the hero's friends and family, or whoever cooperated in taking down the dark lord react to this choice? If the dark lord is in a weakened state, some secondary character may very well try to end him on its own. Should the hero stop them?
Hope I've been useful.
If you want to bring this villain back at a later time, consider whether you want him to even be aware that he has been trapped in his realm by the hero.
Get him chasing his own tail, or have the villain leap to conclusions when he misidentifies the hero as the minion of an old enemy. This gives the Elemental more of his own character and his own worldbuild-y concerns.
This works for an underpowered hero or a trixter, which is usually more interesting than an over-powered hero who wants to be a benign "Dudley Do-Right".
An overpowered hero could just put the Elemental in a chokehold until he passes out, then chain him up. There isn't much jeopardy or suspense in that version though.