I think OPs of a thread are to lock the thread whenever it reaches a certain number of pages and his question is answered already. If anyone does have anything else to say, he can create another thread for his issue.
You have a very good point because that's what causes spamming because often times when a thread is created and the OP intentions is to get good answers from different users as soon as the question of the OP is being answered you see most users almost saying same thing over and over again so if an OP has achieved the level of information they requires in a thread, locking it just as you said is the best option in other to avoid spamming the thread.