I have to throw caution here.
Be careful with direct deletes like that. If one of the replies is listed as the "last post" in a forum, and you manually delete it. The whole forum will disappear from the main index.
Use extreme caution when manually deleting stuff from the database, often other tables will need to be adjusted as well.
If you delete posts, you'll need to recalculate the thread/reply total in the boards table, and if you're deleting replies, you'll need to readjust the number of replies in each thread, to avoid the dreaded "blank page issue" which happens when those counts get off.
Usually to delete anything in threads, multiple tables will need to be updated.... which is why it's not usually wise to use a MySQL command. You can have bigger problems later.
