I usually contact the company directly to let them know when this happens. In all cases a hiring manager has thanked me because they were unaware of these tactics and were happy to get rid of this contract.

My worst experience was a recruiter who would send me an email every day and call me 5 minutes after sending it. Literally every day. Even though I never took her call she would try again the next day.

It went on for 2 full weeks before I lost it and finally replied to one of her emails with a simple "Stop calling/emailing me" and it stopped.

heh. And here I am feeling guilty when I will occasionally send a single follow up now and then.