Gregory's ServerI advise to be careful with quick defederation (except maybe for instances where you don't have any connections with).
All connections (follows) between your instance and the blocked instance will silently disappear, and people even won't even notice.
It's really hard to rebuild the connections afterwards and some peers will be lost forever.
This spam wave is the result of admins having open registration. Although it is nice and welcoming if people can set up an acc on your instance instantly, it opens the door for spambots.
So please, dear admins: switch to account request "appproval needed".
This will save us a lot of headache.
(Not a solution for the bots that are already present in the current wave).
@earthworm I think another way to fix this is as described at the bottom to block many temp E-Mail providers! But yes, approval required (maybe just for TOR IPs) is a good idea.