For me it depends. If it's a duplicated topic then I usually keep it locked so that if the creator of the topic comes back to check it, they can visit the link we usually give. If I don't leave a link, I usually leave instructions as to where the first topic was created so they know where to go for it.
If a topic is ridiculous, like the title is "SEX" and the first post is "i am sexyyyyy" or something ridonculous like that, then I will delete it.
i agree with babs about the topics that get deleted, the other day the title of the topic was "f**", i mean come on what a stupid topic, ryan smyth, the twins and joe sakic were some of the choices