Last year, U.S. lawmakers succeeded in pressuring Belgium-based SWIFT electronic payment system to block Iranian transactions.

CENTERFORSECURITYPOLICY: Center for Security Policy | US Congress to Target European Central Bank to Stop Iran from Using Euros

There are no documented cases of bank employees using FIX vulnerabilities to block financial transactions.

FORBES: Hacking Capitalism

The UN's Secretariat was charged with its execution, but the Security Council maintained a sanctions committee that reserved the right to block any transactions requested by Iraq.

ECONOMIST: A fair-minded report at a crucial time | The

Block financial transactions with tax havens, to stop tax evasion.

FORBES: Will Warren Buffett Be Endorsing Jill Stein For President?

More dramatically, many merchants block all transactions from certain countries.

ECONOMIST: Computing

The sanctions are partly intended to block international bank transactions with Iran.

BBC: India and Iran reach oil pay deal despite sanctions

Britain was seeking to block a financial-transactions tax.

WSJ: Fiction of the EU Summit's Accepted 'Facts'

E-businesses using anti-fraud software now block about 8% of all transactions.

ECONOMIST: Computing