[TomGullen]

CAPTCHA's did nothing to prevent spam for us.

However simply disabling users < 3 posts being allowed to post hyperlinks got rid of 99% of spam.

We initially allowed plain text links but spammers seemed happy to just post them.

Spammers want to post hyperlinks, without that ability they can't do anything.

[borplk]

Just an idea:

Make links remain as plain text (so they are plain text as far as google is concerned)

then use javascript to turn them into clickable links when the user mouse is over or near the link so the user can still click on them.

[TomGullen]

Spammers still relentlessly spammed us when we turned them into plain text links. Removing the ability to post plain text links was the only thing that stopped them.

No idea why they seemed happy to post plaintext links but there you go!

[prawn]

I do something similar which has significantly cut down on my spam too.