Maybe give the users the oppertunity to say wether they accept one or more of the posts as the answer to their question/problem or that they have the solution to their problem.

This would most probably be just a normal "Mark as locked" thread but in a more "User Friendly" manner.

This will also help in terms of
1. Housekeeping (From a supporter/moderator point of view) so it's known which threads is still on-going (wether it has become a "chat" thread or no one has ever had an answer to the specific question).
2. Expand the search facility to allow a search on only "answered" questions/problems.

The downside to this however, is that others can not reply and say that they also have the same problem and that they are glad to hear of the solution and/or that others has the same problem, unless you can make a change so there is an extra flag of "Answered" and not necissarily "Closed" on the thread, which would still allow for "Thank You" posts and/or allow the thread to "mutate" into a "chat thread"