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 (recommended, as it carries about 80 rockets). Fly to the city of Merida and shoot the billboards. You will earn ,000. If you shoot enough of them you begin to unlock more shop items from the P.L.A.V. and a few 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 those pesky billboards.