DefaultTrust could be overrun by a 51% attack.
If that would be the case, then signet would be flooded by 51% attacks. But because blocks are signed, it is impossible to abuse that kind of network. Which means, if you have a good blacklist, and whitelist, then most of the traffic is ignored, and only some messages are included.
Also, note that at the beginning, there will be little or no traffic, and any tools for censoring content, will be introduced gradually, when network will grow. Because you may not remember those times, but it was possible in the past, to write posts on Bitcoin Forum (that was not yet called "bitcointalk", this name was formed later), without logging in. And it was disabled later, when Bitcoin became more popular. In the same way, decentralized forum can be formed: during the first days, it is unlikely to be flooded, if something is new and unpopular. It is more likely, that people will ask questions like "how to connect to decentralized forum?" or "how to sign a post?", in the same way, as they asked "how to unlock port 8333 on my router?".
