As for alts, there could always be a time limit such as 1-2 days before new users are allowed to post
As for short posts, the forum can already tell you that your post has too much quoted text, so why not to little text in general. Something like 15 letters are required.
As for spelling, there could be a built in spell checker for users with trouble (or just use the Google toolbar).
As for the general intelligence of the posts, there doesn't seem to be a solution in sight at this time.
A possible answer is a crackdown-day that will come unannounced. All the mods will be informed. Their names will be hidden in the "who's online now" section. The date will be worked out so that the possible amount of mods can be active at one time. They will be allowed to ban as they see fit. Permanent bans or even account deletion, IP blocks could be used.
This is a very harsh solution that some users will deem unfair. Another flaw, is that it's fully depending on the judgment of mods. Users disagree with the decisions that they make on regular days so I am sure it would be hectic on the Ultra-Ban day.
One final solution is to hide the mods names all together. Instead of tinting them green in the who's online now area, there names could be just like all the others or possibly hidden. Still I notice the same amount of junk weather there are moderators online or off.
The last two answers would cut down on Spam. The reduction of Spam could attract more intelligent posts. I believe that it may, but most likely there will still be the same shit "not Spam but damn close to it" topics. Sorry I can't offer a sure-proof solution. Maybe more of these “the BBS is going downhill” topics will make users think before they type but it’s a long shot.