If you have already tried to rally the server, and destroy these fortifications, and that effort has not been effective, then it may just be time to find a different server.
The public servers will live and die by the toxicity of the players on that server. If a clan gets to big, and too alpha, eventually they will have nothing “fun” to do because they own everything. And because they will have chased all of the other players off. They will lose interest and leave, or work to take on another server. Then their stuff will decay, and eventually open up again.