A bitcoin sign with a graph pictured in the background. STR | NurPhoto via Getty Images I’ve noted in past commentaries that sovereign nations have the power to regulate and eliminate any competitor to their own currencies. One country took that step on Friday . China’s central bank has just made all cryptocurrency-related activities illegal. The value of bitcoin plunged over 5%, while other digital coins are also trading lower on the day. What China did may well be repeated in other countries. Regulators in the U.S. have already expressed some notable disdain for replacing the U.S. dollar, the reserve...