I am fed up of contributing to discussion threads only to have the thread creator silently delete the entire thread and thereby waste my time and effort.

The fact that Fimfiction allows users to delete their own forum threads is, in my opinion, a poorly-considered feature that should be removed - however, the fact that you can do this doesn't mean that you should. It is flagrantly disrespectful to people who take the time and effort (sometimes, a lot of time and effort) to respond to you. It also discourages interaction - why should anyone bother to comment on a thread that could disappear at any moment?

It is frankly ridiculous that I keep having to explain this to people. This should not need explaining. This is basic common courtesy to your fellow users. Do not delete threads. If you really want a thread deleted, speak to a group admin and we'll decide if it's appropriate to do so.

The next time I catch anyone deleting a thread in this group, they'll be banned.

EDIT: For clarity, we'll be lenient if you're just undoing a quick mistake, although I would still prefer that you request thread deletion from a group admin. Deleting a thread to which people have already contributed discussion, however, is a no-no. If you're not sure, ask an admin.