Their money can't be irretrievably sent across the world instantly. You'd have to hold them hostage for at least a few days, make them sell all their shares and other holdings, wait for those transactions (which may well have tanked some stocks, leading a dozen reporters to your hostage situation) to clear, wait for the money to get into the bank, wait for the bank to transfer it to another account, and hope that at no point in the process anyone gets suspicious that a billionaire is liquidating everything and sending it to Eastern Europe.