Banning your access to legally purchased products by your own money for not saying good things on forum is the least good idea they could think of. And that's what you're proposing that it should be applied here.
Not for not saying good things. But severe cases of abuse might lead to consequences. I admit, that products which were bought probably cannot be taken away though. But still they can be banned from using the forums (as i said, in very severe cases).
I think the point you are missing is that game trading / giving away, is actually, legally, very rocky for GOG. The giving away isn't so bad (I don't think) as it's not resale. However trading is probably going against certain licence agreements that GOG have signed, and that we have signed. I was amazed that there were bluetext posts in this thread, I think it was just because things were getting silly. However the more formal that GOG make rules on this, the more they become complicit in the practice. I very much doubt they want that.
In short: GOG cannot intervene at such a level, we're on our own.