Are their comments worthwhile, but too short? Or are they stupid and they add stupid words to get by?
If those padded comments are useful, then lower the limit, as suggested. If they are garbage fluff, you might just silently delete them. If you want to “educate” them, you can send them as many PMs as you have the stomach for. You could try customizing the “too-short” text to sometihng like “Dude. Your message is too short to be useful. If you add more useless stuff to pad it, it’ll be deleted"l” text to say something that might “educate” them. They probably won’t read it, but if it changes, there is an increased likelihood that they’ll notice it.