If I were dictator, this is how Steem income would be taxed:
The Finnish Tax Authority would have an account on Steem, say @taxfinland, and it would run bots to track all transactions by known residents of Finland. It would calculate the taxes owed according to easily understood and sensible rules. It would send a weekly bill in a message which could be paid in STEEM or SBD. It would power up some of the taxes paid, which would most likely make it a large stakeholder eventually. It would replace @suomibotti and dole out big upvotes to every tax paying account. :)
I would actually prefer an automatic taxation to a weekly bill.
Impossible to do on the Steem blockchain unless you authorize a bot to transfer money from your account through SteemConnect. I'd prefer not to do that. But maybe a smart contract on SteemEngine could do that in a decentralized manner. But if you get a bill, at least you can check it first for errors and not pay until possible mistakes have been sorted out.
I so much agree with this comment. One of the things most people enjoy on the platform is that no-one except them has control over their account so, an automatic taxation would defy that.
As much as I would have loved your idea, it's so painful my country's eyes are not yet open to the blockchain world.