If you don't want people making one-word posts, add a rule against it. It's a common rule and nobody would complain.
That wouldn't be the way to go about doing this, though. It would be complicated because there's too many situations where a one-word post would be legitimate and would have a lot of thought go into it. The issue we're trying to fight here is when little thought is put into a post. But not all one-word posts are pointless, and not all pointless posts are one word.
It seems then that an appropriate rule would be something like "if you don't have anything to add, don't say anything." But that's more of a guideline than an enforceable rule; don't make inane posts.
I can't think of any good
rule.
Actually, this reminds me of a forum I visited before... I think it was Pseudolonewolf's forum. Anyways, whatever forum it was, it had quite an interesting set of rules, and there was community moderation. A post could be downrated for being too short, too long, sounding too stupid, being too confusing, having bad grammar, having no point, or several other bad qualities (and then a banner would appear on the post like "Points were deducted for: Brevity")... Posts could also be rewarded for good qualities, too. It was nice that there was a forum where your user rating reflected the quality of your posts...
But I wouldn't like to see rules like that anywhere else, because they definitely had harsh criteria and they were very subjective. Even though all the rules were consistent with my regular writing style and I had no trouble following them, I was in constant fear of being downrated.
(edit: Yup, it was Pseudolonewolf's forum. It's called Fig Hunter, and
here are the rules. Scroll down to "Violations" to see what I'm talking about)
I do think that posts with little thought put into them should be discouraged, but I think that any rule
disallowing them (or anything of the sort) would be too harsh.