The zone chat is atm just being used as a second trade chat with more visibility because many people don't want to be in the trade chat, so it's just abused as trade. It'd be nice to see it actually being used as zone chat instead of a second trade chat for people who want to bother the people who don't want to be bothered with trades.
I completely agree with you but I can already tell you the solution won't be elegant. I've played many MMOs and this is an issue in Every. Single. One. The ONLY game I've seen try to come up with a remotely functional solution is FFXI which has a customizable chat filter you can use on every chat log channel, however the labor of creating the filters is left entirely up to the player to manage. (Tangentially: Kinda shocking such an old game is the only one I've seen have any success tackling this)
If GH tried to moderate it themselves they'd find themselves quickly overwhelmed. Even with the game being as low-population as it is, we'd still need 24/7 monitoring on the zone chats for every instance of haven. Even just a single trader is capable of forcing that.
In the case of chat filtering, traders can be frustratingly resilient. I see that filters can be circumvented within a few weeks by just changing how they format their messages. It's really baffling, but it's just the reality I've observed. Some traders will do everything they can to circumvent your chat filters even though having filters means you very clearly have no interest in their wares. Though player managed filters could work well, you'll need to accept that there's no guarantee you'll never see an ad again where you don't want to and the labor will be your responsibility.
Ideally, people would circulate their own filters for others to use which is what happened in FFXI. It probably won't be as much of a headache as I've portrayed it to be.