What your options are depend a bit on the company, its pecking order, and your place in the pecking order. Does your boss have a boss that will listen to reason? If so, talk to him/her. But make sure you can argue your point. You should be able to clearly demonstrate this action will actually hurt the company. Otherwise, you only have a few options left. Do what your boss says you should do. Quit. Or refuse to do it and get fired.

Of course, if it's illegal to send spam in your jurisdiction, you can refuse to do it, and take legal action if they fire you for not doing your work.

What you should NOT do is to sabotage the sending of the spam. That would just give them to many hooks to fire you (and maybe even sue you).

Of course, the most ethical thing to do is to quit.

Abigail