Simple suggestion
When your bounty exceeds your insurance rebuy cost you lose insurance.
When your bounty exceeds the cost of your ship the bounty comes out of your net worth and may result in the loss of stored ships as well.
Problem solved.
You want to be a serial killer? Fine. Pay the price when the man come to get you...
The main point that many are missing is that the current system allows for huge potential accumulated gains with no commensurate accumulation of risk under the current mechanic. While the bounties are rising to astronomical levels, the offender still is risking nothing more than the rebuy cost.
My suggestion is merely to balance the equation. Racking up huge bounties is fine as long as it comes with a huge cost.