Unlock the P.L.A.V. faction and get a gunship with rockets, such as a Castro V Attack chopper or an Anaconda from the pirates (carries about 80 rockets). Fly to the city of Merida and shoot the billboards to earn $5,000. If you shoot enough of them, you start to unlock more shop items from the P.L.A.V. and a couple things from the U.P. mercenaries in their city. Only one rocket is required to destroy a billboard. It only takes 1 rocket from a gunship to destroy the billboards.