Just a thought or a suggestion for you Dani et. al. If the last post is older than X number of days old (say 30), remove the reply box and in its place, put a button/link that says something like "Would you like to start a new thread that references this one?", and of course they would be taken to the post new thread page for the forum and the first line in the edit box would be, In reference to "This Thread:Thread Title Name", which of course would actually be the BB code for the link back to the thread being referenced.
Bonus... Only the OP can resurrect the thread... (like I'm about to do... that is if I can find it...)
Just a thought...
EDIT: During my search I see that this has been brought up before... http://www.daniweb.com/forums/thread265375.html