I repeat my statments with other words.
If IP.Board is like other forum providers, then locking threads of your own just can't be done. Why?, because that permission goes embbed to an entire section of the forum, not own threads only. There's is a very small chance (just error margin, but I do believe there's no chance at all) that IP.Board allows to set this permission.
Besides that, closing threads is ALWAYS a job for a moderator, and if everyone feels like common users should be able to do it aswell, then the correct solution is add more forum moderators.