the permissions profile would certainly be the better way to go... that's how I have my announcements board set: read only for all users except admin. (and that even stops global mods from posting to it - they could normally break a post lock)
Additionally, using that mod, you could set the "auto-lock time" to 1 minute for the target board... that would handle specifically what the OP is asking for as well - any thread which was moved into that board would be locked shortly after being moved there...