Everything about Lemmy; bugs, gripes, praises, and advocacy.
For discussion about the lemmy.ml instance, go to [email protected].
There isn't and Lemmygrad.ml users have been doing it in the past. Mostly down-vote brigading though and this is the main reason why down-votes are disabled on our instance.
Personally I think there should be a setting to prevent down-votes from federating as there is really no point in federating them to instances with other rules and different community culture.
Malicious instances would need to be blocked by the instance you are registered to.
As for multiple accounts - couldn't the same thing be done on reddit? Just make a bunch of alts and self-inflate your own posts/comments.
These issues are as old as the internet and can only be mitigated, not prevented.
Reddit has some kind of detection for that and you will get banned for using alts to vote your posts up.