Well you cant really but you can do something nearly as effective-closing it
you change the name by clicking edit on your first post of the topic
then you change the name to CLOSED or have [closed] in brackets or whatever you want nexto the name....nobody really wants to post in closed forums unless they are stupid, alsoyou can delete the old comments so nobody will want to post
hope that helped smile
If you are a mod of a guild near the bottom there will be things about closing or locking the topic. There will also be things above individual posts giving you more deletion control. If it's general forums then you can't delete. Though kind of you to try to help with Gaia's server space the mods will eventually delete topics when they are truely dead or taking up space. If your having trouble with subscribed threads the button at the bottom turns into an unsubscribe button on subscribed threads.