I wasn't suggesting that the Moderators "manually" review all posts and/or follow up those that are not marked as solved - that would be crazy in this electronic world. Nor do I deny that better solutions may be offered at a later date. I also wasn't suggesting the thread be locked, so interested users could still post to it even if the thread was marked as Solved. Remember that users can see the date of the last post, so can get an idea as to whether or not the thread is "current".
Instead, I was thinking that this issue could be solved (pardon the pun) programatically with a question ("Problem solved?") in a pop-up dialogue box (rather than a checkbox on the Post Reply form - which relies on the user seeing & going to it) presented to the OP when they submit any subsequent post to that thread. If they answer Yes then SOLVED could be added to the thread subject line. As this would force the OP to respond, I would expect a much higher strike rate with this method than anything else, and little - if any - downside.