Some of us are simply against capital punishment, period. Others are willing to compromise in certain cases – serial killers, evil dictators, the guy that raped your sister. Even to those willing to compromise, even suggesting capital punishment for spammers might seem outrageous. After all, spam doesn’t kill. Spam is annoying, but so is traffic, right?
But notice that the guy who raped your sister is causing more serious harm than a spammer, but at the same time, he is only harming a very small number of people. A spammer, on the other hand, is causing a very minor annoyance to a lot of people. Like: a lot of people. Anybody with a computer, basically. And not just once, but everyday. The annoyance adds up. It might seem that the spammer is causing as much harm as the rapist, albeit distributed over a large group of people.
Now imagine you’re taking something like the second attitude towards capital punishment, and imagine you are somewhat of a consequentialist, so you care primarily about outcomes of actions when you evaluate them. What is your reason for defending capital punishment in case somebody raped your sister, but not for spammers ?