Unidan could just create an account on multiple instances and vote for his posts/comments with all these accounts. That way his content would gain more attention than those of sincere users.
In case of malicious bots (like those annoying bootleg bots on reddit), it might even be profitable for them to create their own instance(s) just for that purpose.
Is there a mechanism to prevent that? (other than user/instance banning and the introduction question on user creation)
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.