Dear Simon Rumble,
You'll fight a losing battle with the spammers, especially if you use off-the-shelf software with >5 other users. Even with something you write yourself, like my my crappy script you need something non-standard to keep the spammers away.
No. Wrong. I'm using Drupal, one of the most popular CMS packages on the planet. Do you see spammy comments on my site?
Sure' there's a bit of attempted spam. The latest batch of three from this morning are sitting over in my admin block, waiting till I get around to deleting them.
Actual spam getting through though? Okay, so out-of-the-box it does, but all I added was the standard spam module and I've not had a problem since. Don't even have a captcha!
Turning on comments doesn't gain you protection from spam; I've got them on and don't get any. Turning off comments *does* make it much harder for people to respond to your posts, and does ensure that comments on your posts can't be made in the open (at least, not by emailing you, which you suggest as a means of contacting you. That's why I'm blogging here, because this comment is intended to be public).
If you don't have comments turned on, you only have half a blog.