Interest free banks would be fee based to cover their accounting and profit. You could break it out to a per month percentage cost ex. 1% - 3% per month but the days of usury would be over.
Sure banks are necessary but not useful when trying to control the political arena esp. on a worldwide basis.
So people who own banks wouldn't be "allowed" to charge interest under your administration, however it would be OK for them if they charged 1% or 3% per month. :)