Beehaw is run by 4 admins. It's a very closed instance. You have to apply to have an account. Only the moderators can create communities. They have strict rules about what can be posted. The 4 moderators can't keep up with the comments that are being posted to their communities from federated instances. Because some instances allow anyone to sign up and create an account a troll can create an account, and post harassing comments in Beehaw communities. Beehaw can block that account, but the troll can just delete that account and start a new one.
Blocking the instance as a whole was the only option they have at the moment. They will need to make a decision on how they move forward, either by adding more moderators or continuing to block instances. The two they blocked aren't the only instances with open registration. Plus, since you can comment from any platform (Kbin, Mastodon, Pixelfed, PeerTube, etc.) there are a lot of instances out there that could be used to troll Beehaw.
I hope this is just a stop-gap measure and they will open back up when things cool down, but I'm worried that they just made themselves a target for more harassment.