dok not a fan of locking a topic.. For starters just because the OP think they "solved" their issue doesn't always mean it was, or the info given that they think is the solution was the correct info or the actual solution to whatever their issue was.
If they want to mark it as such and make comment to what they think the solution is that is great, but should be left open for atleast some time to allow for corrections or and or updates to the info in the thread, etc.
Locking threads after X amount of time, sure all for that - keeps people from adding to threads that are years old, etc.