On our busy successful community, we hide the Reply button for old topics in forums that get a lot of buzz and activity. They will see a message that says "This topic is over 365 days old and expired. If you would like to discuss this subject, feel free to start a new topic."
In some of the smaller forums with just a couple pages of posts, we have it turned off. We don't mind bumping something from page 2 or 3 to the top, but when something is several years old and a couple hundred pages back, we would rather it remain dead and buried. Many of those members are gone by now anyway so there is no use to replying to them.