where do you set the options for this?

Under the forum manager. See Here. The same place where you would edit the forum title.

hm, sorry i missed that.

it'd be nice if there was a way to turn it on or off globally though- adding the option for every forum is a bit of a pain.

@Live Wire
This Hack can be implemented without File Edits in RC1 by using Hooks forumadmin_add_default and forumadmin_edit_from, so you might want to update it :)

Great! Thanks man, going to check out the new release now.

UPDATED FOR vB 3.5.0 RC1

First post updated with zip. Zip includes readme.txt and product-exthreads.xml.
Please read the readme file before doing anything!

Couple of new features and cleaned up the code. See first post.

Thanks for all your support!

Nice Hack :)

What i'd like to see:
Users can still reply, but if the Thread is expired they will see a Warning (Maybe an Image and some blah blah Text saying that the Thread is pretty old) and have to tick a Checkbox that they are aware of this but still feel that it's necessary to answer.

That's because sometimes it does make sense to bump an old Thread, but in most cases it does not make sense and the Users just don't realize how old the Thread is.

Btw: Moved to Plugins :)

Never thought about that. I suppose I could do that as well. I'll just make it an option to turn on/off. Note to self: Physically lock expired thread would have to be turned off for this option to work.

My setting look like this:

You need to use Import Products and not upload it as a plug-in. ;)