I cannot force them to upvote articles from newbies, even if I think it would help STEEM/STEEMIT to grow ...

I myself try to upvote - besides family and friends - also small accounts.

Here are some ideas one could think about to implement to make self-voting, circle-voting and spamming less attractive by ...

• ... thinking about a reward curve which started as n^2 / exponential (thus flat), and then later changed into linear which would work against self-voting as well as excessive rewards.
  @clayop had a similar idea.

• ... implementing diminishing returns when upvoting the same accounts (including own ones) again and again.

• ... reintroducing the restriction to four (or less) full paid posts per day (from some hard forks ago) which was very reasonable.

• ... considering also other ideas like the one of UserAuthority from @scipio.