Billy Bragg threatens to withhold tax in banks protest

The BBC is reporting that songwriter Billy Bragg has threatened to withhold his tax payments unless the government forces banks such as Royal Bank of Scotland to start paying back the taxpayers' money before they give their employees bonuses. Bragg has launched a Facebook group explaining his position:

If you pay tax in the UK, you are a major shareholder in several banks - Royal Bank of Scotland (RBS), Lloyds and Northern Rock - who were saved from financial ruin by government intervention in 2008.
The Treasury spent £117bn of taxpayers' cash bailing out Britain's failing banks - and last month it was revealed that this figure could rise to £850bn.

I understand that the Treasury had little choice but to use taxpayers' money to safeguard savings and stabilise and restore confidence in the financial system.
What I don't understand is why, now that we taxpayers are the majority shareholders of these banks, we seem totally powerless to curb their excessive bonus culture?
The estimated £1.5bn that RBS will pay to its investment bankers next month in the form of bonuses will ultimately be drawn from the taxes that you and I are due to pay on 31st January.
I have written to the Chancellor of the Exchequer, Alistair Darling, to inform him that I am no longer prepared to fund the excessive bonuses of RBS investment bankers. Unless he acts to limit them to £25,000, I shall be withholding my tax payment on 31st January.