I mean the easy solution is to have your instance enable registration screening. That's what it's for. It takes less than a minute to answer most instances' registration yet (based on experience) that simple barrier is enough to stop most trolls.

Until a Lemmy instance gets large enough to actually hire full time admins to catch and remove abusive content ASAP, I don't see instances reasonably being able to go without registration screening because the trolls will seize on that opportunity every time.

Admins of larger instances see it all the time:

1. New instance pops up, yay! And most instances automatically federate with new instances!

2. It doesn't have registration screening, this is quickly discovered by trolls and adbots and the instance gets filled with rule breaking content.

3. Large instances start blocking it because by federating with an instance that is being used in this way degrades the quality of your own instance and adds a ton of workload to your (unpaid) mods and admins.

4. The instance eventually enables registration screening, and other instances start unblocking it.

It's happened with plenty of instances before and will probably keep happening as long as spam and trolling exist.