Custom ranks: User must be registered for X amount of months in order to use it and thus can't abuse it (you can abuse postcount - you can't abuse time).

Avatars: You put up a select group of avatars and disable custom ones. This way people can't abuse it (cept admins and mods...and i doubt they would).

Just some ideas but they work pretty well from what i've seen.
One of the best/worst ways for spamming is to make a spam section. So as long as you steer clear from that and moderate the board well enough then spam should be a minor problem rather then the large one that you people think it will be.