Tobin tax. Now. Go read about it.
"you'd like to charge everyone a flat fee to conduct a transaction, "#
The "financial services" sector takes a cut of a couple of percent every time Joe Public moves some money around, and the world hasn't stopped because of it.
Debit card payment to retailer? Couple of per cent, no problem.
Credit card payment to retailer? Couple of per cent, no problem.
Credit card balance transfer from one card to another? Couple of per cent, no problem.
Mortgage balance transfer from one provider to another? Couple of per cent, no problem.
Advice to Greek government on how to hide state of Government finances off books ? Couple of per cent, no problem (whoops, wrong example, never mind).
Tobin tax at a tiny fraction of a per cent when money moves from one rich man's City institution to another? End of the world!
It's a zero-sum game. If most CIty trading and traders really "added value" overall (the usual "liquidity" myths etc) there'd be sensible ordinary folk outside the City willing to defend them, in the same way as there are folk willing to defend the amounts we pay footballers etc. No sensible outsider defends the City as it is today, because they don't "add value" overall.