I was just thinking about this, and please just ignore me if that doesn't make sense or you have thought about this already (my students usually ignore me, so why shouldn't you...):
Isn't there a way in such a case to give people some kind of grace period, at least if the dialogue is not initiated by the respective person? ? Something like 'I see you are a knifemaker and you entered here in a discussion about your knives. We welcome your input, but please be aware that we only allow a 2-week grace period after which you are required to decide if you want to become a paying member or stop contributing anything that falls under the vendor privilege.' I have a feeling that many of us would welcome input from other knifemakers and appreciate a dialogue, and I found the - what appears to be - immediate ban in this case a bit harsh. Just contemplating how we could invite them rather than brushing them off. Of course, anybody who actively comes in with 'Here are my products, go buy them!' would still be SPAM.