We have discussed this, but it's tricky. For instance, I considered looking at calling our IPS Spam Service for emails sent through the contact form, but we decided that this couldn't be done because a valid user who was blocked from registration by the spam service would then have absolutely no way to contact the site admin to let them know or get assistance.
Unfortunately, if you leave a contact form in place, spammers will use it. It's tricky to block spam effectively here without resorting to "hacks" (like scanning the email for specific text which may not actually be indicative of spam).