Legally speaking, billboards are only banned within 500 yards or some other distance from the highway with the most traffic (where billboards would be most valuable, namely, US 101) but actually there are no billboards anywhere in the county and this has been the case since the 1960s (according to an old newspaper article). My guess is that the community has some way to exert "informal" (not based on formal governmental processes) pressure on landlords. Real estate prices are very high here in part because it is a very attractive landscape with plenty of hills and greenery and bodies of water, so maybe most landlords perceive that billboards have the potential to depress prices and keep the occasional landlord who contemplates erecting billboards in line somehow.
Also as an exception to the general rule, bus shelters (structures owned and maintained by the city or the county to keep the rain and the sun off people waiting for a bus) near US 101 have ads on them (4' by 6' or so) and the buses themselves do, too, or at least they used to--it's been a few years since I noticed.