Pretty sure someone has corrected you on this point already, there is no single central world bank that regulates currency, as such all major governments would have to be on board to destroy the use of cryptos, in any case your proposition is baseless, there is no way for you to know governments are going to ban bitcoins.
"Give me control of a nation's money and I care not who makes the laws." Mayer Amschel Rothschild
1) Method 1:
Prohibition of alternatives and showcases"The Liberty Dollar (ALD) was a private currency produced in the United States. The currency was issued in minted metal rounds (i.e. coins), gold and silver certificates and electronic currency (eLD). ALD certificates are "warehouse receipts" for real gold and silver owned by the bearer. According to court documents there were about 250,000 holders of Liberty Dollar certificates.[1] The metal was warehoused at Sunshine Minting in Coeur d'Alene, Idaho, prior to a November 2007 raid by the Federal Bureau of Investigation (FBI) and the Secret Service.[2] Until July 2009, the Liberty Dollar was distributed by Liberty Services (formerly known as "National Organization for the Repeal of the Federal Reserve and the Internal Revenue Code" (NORFED), based in Evansville, Indiana. It was created by Bernard von NotHaus, the co-founder of the Royal Hawaiian Mint Company.[3]
In May 2009, von NotHaus and others were charged with federal crimes in connection with the Liberty Dollar and, on July 31, 2009, von NotHaus announced that he had closed the Liberty Dollar operation, pending resolution of the criminal charges.[4] On March 18, 2011, von NotHaus was pronounced guilty of "making, possessing, and selling his own currency".
"Attorney for the Western District of North Carolina, Anne M. Tompkins, described the Liberty Dollar as
"a unique form of domestic terrorism" that is trying "to undermine the legitimate currency of this country".[28]
http://en.wikipedia.org/wiki/Liberty_Dollar2) Method 2 (soft force):
Killing off media of exchange competition via capital gains tax law:Germany:
"After a parliamentary inquiry, Germany has clarified its position on Bitcoin. The federal ministry of finance recognized Bitcoins as “units of account” — the cryptocurrency is therefore be considered as “private money”. That’s why, according to Die Welt, the
government stated that there wouldn’t be a tax exemption on commercial activities that use Bitcoins. In addition to
paying capital gains tax, Bitcoin users should now pay sales tax (VAT) as well."
Capital gains tax in Germany: 25%
http://techcrunch.com/2013/08/19/germany-recognizes-bitcoin-as-private-money-sales-tax-coming-soon/US:
"Did you know that almost everything you own and use for personal or investment purposes is a capital asset? Capital assets include a home, household furnishings and stocks and bonds held in a personal account. When a capital asset is sold, the difference between the amount you paid for the asset and the amount you sold it for is a capital gain or capital loss.
Here are ten facts from the IRS about gains and losses and how they can affect your Federal income tax return.
1) Almost everything you own and use for personal purposes, pleasure or investment is a capital asset.
2) When you sell a capital asset, the difference between the amount you sell it for and your basis – which is usually what you paid for it – is a capital gain or a capital loss.
3) You must report all capital gains. "
http://www.irs.gov/uac/Ten-Important-Facts-About-Capital-Gains-and-LossesAlthough there are some offshore bank accounts that advertise as tax havens,
U.S. law requires reporting of income from those accounts, and willful failure to do so constitutes tax evasion.http://en.wikipedia.org/wiki/Capital_gains_tax#United_States