Unfortunately, there is not much that you can do about it. My practice has always been to post the best possible answer to the problem, maybe some day the user will come back and upvote/accept.
While we'd like the OP to do these things, we want to provide a solution that could be helpful to anyone with the same problem. It's entirely likely that someone else will search for something similar and your solution will help them. Then, they will upvote your answer.
I've had answers accepted years after I've posted them. It happens, and when the late accept happens it's like a nice little gift.