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.