The only way around that is to use the /who API, and since that's not instant, filtering the message directly won't work.
Personally, I've been filling BadBoy's custom filter list. When I see a new ad, I just add the guild name or some identifiable part of the ad.
If you're curious, this is my BadBoy list:
Code:
BADBOY_CCLEANER = {
"(selling)",
"<aspirants of asgard>",
"<calculated chaos>",
"<exordium>",
"<ez>",
"<grey parse elitests>",
"<immortal>",
"<infinity>",
"<lose your life>",
"<mature content>",
"<metacortex>",
"<notion>",
"<obscure radiance>",
"<obscureradiance>",
"<relapse>",
"<reload-dalaran>",
"<reload>",
"<ricos roughnecks>",
"<the fort>",
"<welfare epics>",
"[chaosity]",
"for more info",
"group selling",
"heroic argus only",
"hollow point marshmallow",
"kirby carries",
"nfa realm first keystone",
"raidtargetingicon",
"selling mythic",
"selling antorus",
"selling daily",
"selling full clear",
"selling heroic",
"selling mythic",
"selling normal",
"stormseekers",
"where fun and autism meet",
"wts antorus",
"wts argus",
"wts h antorus",
"wts h argus",
"wts heroic",
"wts mythic",
"wts normal",
"wts quick heroic antorus",
"wts uldir",
}
The best one is "raidtargetingicon" which catches all raid icons by matching the text that formats into the icon.