Oppose it on cost/benefit grounds -- the benefit is (hopefully) quite low, ideally there will be no sales that can be attributed to this; and the cost is potentially quite high, the cost can easily be approached from a Fear/Uncertainty/Doubt angle.

Higher-ups in the company will get very concerned if they hear that the company's reputation is taking a hit (even a small one), custom could be being lost rather than gained, and that negligible benefit has been realised. Ensure that your boss is kept aware of the complaints received, either from end receipients of the spam or from ISPs; even if he isn't concerned that what's being done is wrong, he will hopefully appreciate the negative impact on business it will have. Alternatively, what do the 'big servers company' think of this? Potentially your local ISPs could start blocking ranges of IP addresses, which could *really* concern your hosting company.

Sabotage is a dodgy way to go, I'd be inclined to (1) ensure he is aware that you think it's a bad idea, and (2) do exactly what he says. Not a good way to go, but so long as your concerns are recorded, it's probably all you can do given that what the company's doing probably isn't actually illegal.

Good luck.