Oh... and before anyone suggests it... I know that in the game you can go to the League News, select "Write/Edit news" and then delete every story one-by-one that way.
Of course, when you do that, you have to scroll to a news story you want to delete, click on it, click "Delete Article" click OK on a "Are you sure?" confirmation, and then scroll back to the next news story you want to delete because it automatically scrolls you back to the top. With three years worth of stories that I want to delete, it should only take me a couple of hours.
I'm still hoping there is a better way.
Although if anyone is interested, if you delete the old stories you don't want out of the "messages" folder and DON'T delete the messages.dat file, the game loading and saving time is much, much quicker - you just have a bunch of empty stories at the bottom from all the "messages" files you deleted. So if your goal is to keep the last few months of news around but improve league load/save times, and you don't mind having a bunch of empty "placeholders" for the stories you deleted, than this method might work for you.