I'm a firm believer in the idea that the rules of a forum should service the needs of the forum. Activity that is problematic on boards with thousands of active members isn't necessarily going to be a problem on a board with fewer than 100. Obviously it would be bad if the forum filled up with stupid, pointless, annoying messages. But at present, that doesn't seem to be a real risk. We have a small member base sharing some lighthearted fun. An odd post here or there which might technically be defined as 'spam' isn't going to harm anyone's forumgoing experience. But heavy-handed spam moderation will.
Spam rules should be moderated with a light touch. Threads don't need to be locked or deleted, they can merely be allowed to die naturally. Nobody needs to be called out in public, but a private message that says "Hey, your post broke the rules. Here's the rule about spam for your future reference: ~~~~~" would be effective. I think the rules would only need more active enforcement if there were an active problem.
As to the definition of spam itself, I think you've hit on an important point, Kaynil: "Not all short posts are spam." Sometimes a single sentence, or even a single word, can be a vehicle to drive conversation forward. If we get it into our heads that short messages = spam, then we create a highschool environment. The kind where you finish your essay in 1 page, then have to write 2 pages of bullshit because the assignment was for a 3 page essay.
If I had to write the rule on spam, I might phrase it like this:
[INDENT]Members should avoid posting spam messages. Spam is any post which does not contribute to conversation. If you can't imagine other posters responding to your comment, consider not posting it. If you don't have anything to say, but you want another member to know that you read their post and liked it, consider using the 'like' button instead.
[/INDENT]
More than anything, I think we should bear in mind that it's more important to make people feel welcome here, than it is to maintain some strict ideal of utopian forum order.