I think it has all gone too far.
I think stop the policing and just come up with a donation page or something, let anybody say whatever they like, if someone is making money give a donation to the forum, have an honesty box type policy, but keep an eye on it, if someone is making thousands and gives you a quid that is obviously wrong.
I don't think banning free talk is ever a good way to run a forum.
if a few people get the hump with it so what ?
If I see a blatant plug I look at it then decide for myself whether it is a load of rubbish or not, surely others could do the same?
as it is nearly always amateur stuff most people will give you plans for their ideas, and if you simply are not able to make the item or just cant be bothered, they will do it for you frequently for the cost of the raw material.
if people are really abusing it then tell them to stop or ban them.
if others are recommending it then it cant have been such a bad transaction can it?