Well, there was a bit of drama last night around 5:30-6pm, with dozens of troll posts being posted on lemmy.nz as well as other instances. Luckily @idanoo@lemmy.nz has emergency access and managed to get it under control elegantly and skillfully.
For the moment we have registration applications turned on. Recently we have not had a lot of new users joining lemmy.nz, most joining our communities are from other instances, so I intend to keep the requirement to apply for an account for the moment. If we get an influx of new users (e.g. from something reddit does), I'll try to work out a way to make registrations open, because it's important to me not to create barriers to entry.
I'm also investigating fediseer.com, which is a spam fighting measure where instances guarantee others, creating a chain of trust. Currently we have very few instances defederated, but if we implement this, and especially if we get it automated, then you may see many more instances on the block list, and these would update frequently (e.g. blocking an instance while it's gettting a spam invasion, then unblocking once it's resolved). I haven't implemented anything yet, though.
Fediseer seems to be explicitly only about spam, so in theory we should not see instances blocked for ideology or anything like that. Though there is the ability to censure instances for any reason. I guess I have more to learn.