Depending on the nature of the thread. If the thread was found to be nothing but spam do not lock it, delete it. If you find a certain member is consistantly creating new threads, you should message them as well as lock the thread with a warning that it will be removed soon. I was looking in the Modding section and I saw two separate threads for the Werivar mod, regardless of if the original was locked there is no excuse why there should be two threads for one mod.
I've also noticed that as we've softened the way we handle abuse that it has increased. If someone is abusing the forum rules you are fully authorized to edit and/or remove their posts to preserve some semblance of order. Make sure however that if you do remove posts that you are not biased, remove any and all that continued or encouraged the off-topic discussion. You can usually trace a string of off topic posts to a single post, always remember to get rid of the root of the problem.
Be firm but informative when you're moderating. If you see a problem, don't simply point it out, offer a solution as well, that is productive. Simply pointing out a problem is not productive. That's all from me and sorry I have to be the hard ass I am