check the modlog (there are 7 pages dedicated to him). but basically:

• kept being sectarian (expressing his dislike of Maoists, anarchists, etc.)
• kept posting and commenting in the wrong places (i'd say half of his modlog is just this)
• would crosspost to 10+ communities for every post
• seems to have instigated a lot of the arguments he had here
• was temp/perma banned many times (on different instances too) for all these reasons but despite apologising didn't change his ways
• and may have doxxed someone (check the modlog for this, a mod called him out on it)

but my impression of Pluto was initially positive too. feels weird seeing a fairly prominent Hexbear user be permanently banned, but wow he sounds like a lot for the mods to deal with and banning him seems more than justifiable to me now.

edit: doxxing point from here