This is one of the many, but perhaps most pressing of reasons we need better moderation and admin tools. There is currently no way to stop spam from instances like this except defederation at this time.
Realistically there are far too many small instances which are set up by a single person who’s setting up an instance for their own learning, their own fun, or other reasons but who isn’t a fullstack developer, a seasoned sysadmin, good at sysops, a database engineer, a security expert, and all the roles an instance currently needs with how new this platform currently is. There will always be more instances like this as an attack vector for spam, trolling, and other malicious purposes.
If the goal is to allow people to set up their own instances and we are promoting and encouraging people to do this, we need tools for the larger instances to manage this or we’re going to end up with a community that is constantly in the process of fragmenting more and more and one where measures are taken by the largest instances to isolate themselves from smaller instances which are frequently used as attack vectors.
If you’re a developer, please please please focus your efforts on more granular administrative tools. We cannot whitelist or blacklist instances. There are no tools to evaluate how trustworthy a source is. If there’s a blacklist/whitelist, there’s no way for an instance or a user to apply to a community on another instance. There are plenty of individuals with great ideas on how this might be accomplished, and having access to many different tools and strategies will make this platform healthy.