There are obvious numeric usernames, but also fake orgs with repos for the users to fork and interact with, and a few account takeovers (i.e. someone had signed up for GitHub in 2015 to make a free wedding website, abandoned it, and the account fell into spammer hands). These used to be easier to report.
With collaterals too I presume [1]. I guess I've been the victim of some automated system. They have banned my account without warning or explanation and they've been ignoring my support tickets for about 2 months!