An old Announce I've missed.
By default, Spip implements now a negative captcha in forums and comments.
The principle is simple : add regular text input field in forms, and
hide it with CSS. This field will be used only by bots because they
don't understant CSS.
It's not perfect but it helps reducing comment spams.
related article : http://damienkatz.net/2007/01/negative_captch.html
.Gilles