I think this is a silly suggestion, for the same reason I think virtually every suggested limitation of free speech on the forum is a silly suggestion. Whom is it hurting? Nobody. If you don't like it, mute it.
Also, it is false to say that because Sandgoose gets somebody to post for him on the forum, he is being illegitimately given the right to continue posting on the forum even though he's banned. He's not at all. What that would look like is an ability to post whatever he liked, whenever he liked, on whatever thread he liked.
Rather, what he has in an ability sometimes to convince another human being who does have posting rights to freely choose to pass on a message using their own autonomous right to post. If Draug posts a message from SG, it's because he chose to, and SG can't make him do it. It's no different from if Draug chooses to pass on an article from the London Telegraph or a dream he had last night: Draug can post whatever the heck he wants to.
Ask yourself where this would go. What if SG sent Draug something to post that didn't refer to his identity, and Draug posted it without saying it was from Sandgoose. Would that be a violation too? Would we have to start combing style to see if someone had illegitimately taken content from a banned user?
If so, then not only would it be impractical and ridiculous (and lead to a very easy ability to make false accusations), it would be an incoherent and _per se_ absurd attitude: why should Draug be punished for thinking that some content has value and is worth sharing just because the source of the content is a banned player?
And if not, then you're being inconsistent. If the only thing you want to ban is Draug's ability to post things that are self-attested Sandgoose posts, where's the rationale for that? What if they make a point that Draug wants made, but the (claimed) identity of the writer as Sandgoose is crucial to the point being made? (It's not that hard to imagine such a scenario). What is the benefit of banning such things?
In any case, be clear that it's not Sandgoose's freedom you'd be restricting, it's Draug's (and all the rest of ours too). There is absolutely no need for such censorship.