Geez, the stupid in this thread.
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Realistically they can try block employee access to the data centers like Samrand and hope for a problem.
AWS - unless they are already on it (I think Investec is) there's no way they'll move onto it by Friday. For a good old South African bank that'll be a year or more's planning alone.
DDoS attack is not realistic anyway unless you get help. Your going to need substantial computing power to take a bank down nevermind seven of them.
No, the South African way to do this is with violence, intimidation and physical disruption of services. There will be nothing smart or clever about it. The unions can't even spell DDoS.