"Where does abuse start, where does it stop?"
I try not to consider the topic in particular as moral question (even if it partly is one). If someone upvotes themselves or 'curates' a bad post because that brings them more curation rewards, they do it because it's possible (we are - some more, some less - greedy humans by nature)! :)
I try to find solutions which discourage certain kinds of behaviour which in the long term damage the platform.
Maybe 'diminishing returns' are a solution, but also a bit unfair to those who are very diligent, each article is a very good one and for this reason alone I want to support them regularly.
As long as "regularly" wouldn't mean several times per day (the definition @haejin prefers), the impact of 'diminishing returns' would be rather small. The good thing about 'diminishing returns' is that nothing was forbidden, but with every further upvote, the rewards were getting smaller (however, your voting power when upvoting your 'best friends' would of course recover after a while).
Flags are okay. But who uses them? Most people are afraid of a flagwar and without a "higher instance", it is anyway hardly possible to find out every selfvoter, every votebuyer.
Yes, of course flags are (too often) misused, but at the same time served to control @haejin here and bid bots.
In my eyes it's not the aim to punish every single self-vote etc. but there should always be the threat that it could happen if one does it too often (similar like not every person who drives too fast is catched, but the probability is rather high ...).
How you do it, you do it wrong - we cannot change people and their attitudes.
We aren't living in a perfect world and won't reach perfection on HIVE (even if I think it's worth trying to improve the system). I like your attitude to try to have some fun anyway.