There is currently no "Hide your income" function. But if you don't want to pay realm share, then just make sure there is no Duchy Share. Since the realm share comes out of the Duke's incomes, then make sure the Duke's income is 0. 
For now, I think it is dumped into the general region tax pool.
Are you certain about this?
From what I could understand from Wiki, taxation applies to all incomes of lords/dukes, not only from their share, which makes much sense, otherwise all would be easily obstructed in the way you describe (the way it is currently on).
