Internet companies remove users posting spam all the time.

The act of spamming people is, in fact, illegal. Having an opinion isn't.

Let’s be careful not to conflate legal with ethical. It could very easily be legal to censor all hate speech (Canada and UK have these laws). And I mean government censorship, not we-don’t-want-to-do-business-with-you-censorship.

I was very explicitly speaking of "legal". With acts like the CAN-SPAM Act, a lot of spam is, in fact, illegal, and it isn't by any stretch a restriction of free speech for an infrastructure provider to shut down someone sending illegal spam.

The fact that it's illegal, that is, the government has outlawed that form of speech, renders it very clearly a free speech restriction.

Is it? e-mail spam yes, I don't think posting advertising on websites or social media is per se - still companies act against it if it doesn't meet their own thresholds.