Mostly I just hang up. I still get the microsoft support calls fairly often. One time I let the guy talk me through the process until the hit enter to visit dodgy website. I then told him I wasn't going to hit enter because I knew it was a scam but I wanted to know how it worked so I could tell my customers about it. I thanked him kindly for his help. The stream of vitriol he emitted was quite gratifying. He eventually shouted "You think you're so mart!". I replied, "obviously smarter than you" and then hung up. I had to ignore the answering machine for a while after that, which was even more satisfying.
More recently I've started asking them "Does your mum know what you do for a living?" That usually gets another gratifying tirade of abuse. I hang up chuckling.