Cease and desist letters.

There are many, many people, and companies who operate under the false belief that the CAN-SPAM act does not apply to them; and eg create new mailing lists to blast many people with their spam. Some of these unfortunately includes corps I have business relationship with (looking at you, Google), so "mark as spam" doesn't work well. Cease and desisting their legal department does. I have changed marketing strat of multiple largecorps by being a dangerous professional.