In recent years, this ugly form of visual blight has become increasingly common throughout the valley. These horrible signs are a form of mobile graffiti, defacing and devaluing our neighborhoods, interfering with legitimate parking, creating traffic hazards, damming the flow of water in gutters, and blocking drivers' sightlines. Although these signs are illegal, it seems impossible to get rid of them.
In theory, the Los Angeles Bureau of Street Services takes an interest in the removal of illegal signs. If you have these monstrosities in your neighborhood, you can report them by calling the bureau at 800-996-2489. Typically, you will be given a case number that you can use for follow up in a couple of weeks if the signs are still there. I recommend jotting down the license numbers on the trailers along with their location and the date you reported them.
Unfortunately, it seems that it is not illegal to park an unhitched trailer on the street unless there are signs prohibiting it. (That is a separate issue from the illegal billboards on the trailers.) Very few streets in the valley have these anti-trailer signs. (And, seriously, do we need yet more signs?) Nevertheless, it might be worthwhile to report these as abandoned or stored vehicles (vehicles left on the street without being moved for more than 72 hours) at www.lacity-parking.org/laopm/1_800.htm.