It will not be proper to close any thread only for fighting spammers. They also create new threads. I feel that efforts to fight spam are giving good results. Today I find less spams.
Locking old thread can only stop accessing to it but spammer can start new thread instead of it..so i dont think locking old thread can reduce spammers.
The spammers are directly posting new spam threads, deleting them is the only option.Permanent solution would be to make a sophisticated software program which can detect these spam and block them at the source.The way yahoo mail or gmail or others sort out the spam mails.I, don't know whether it is technically possible.
Agree with Gulshan and others. Locking old threads is not an ideal solution for fighting against spam. I can see 80% of spam in new posts which are popular. So they are choosing the popular posts and not old posts.
However I do see validity in your concern about old posts. There are few old posts that needs to be locked out for their validity or because they are no more in discussion etc.