I seem to remember there being a thread on HN about spam bot scraping for email addresses, and some of them are sophisticated enough that they can even determine if an element has been hidden through rules applied via an external CSS file.

Using a 1 pixel transparent GIF works - also worth putting in a warning for screen readers not to follow the link.

FWIW I've been using robots.txt honeypots on two sites for the past few years to deter scrapers. Approx 1,600 IP addresses have been banned. Most of these are for data centers.

This is what Project Honeypot does http://www.projecthoneypot.org/faq.php#f