Hey,
I've done some permissions updating and it should now work like this:
Users can still delete their own posts, as long as it isn't the first post in a thread. This message should appear if it is attempted:
You are not allowed to delete the first post in a topic.
If you want to delete this topic, click on the Remove Topic link, or ask a moderator/administrator to do it for you.
Previously users could delete their own topic, even if it had hundreds of posts, this now cannot be done.
Hope this helps,
Michael.