It is currently possible, through Lemmy's API, to create accounts automatically and without limit if verification by email address or captcha is not activated. I'd advise you to activate one or both of them NOW!
After registering x number of accounts (currently I could do thousands), all you have to do is list all the existing communities for each of the account to publishes one new post per community, or more. I'll leave you to picture the mess.
(I apologise to the administrators of sh.itjust.works, I should have done the test with my own server.)
I'm surprised captchas weren't enabled in the first place... @[email protected]
Captchas are enabled