Notwithstanding any other provision of this chapter, any automobile wrecking yard or junkyard in existence in any R District and any billboard, advertising structure or nonconforming sign in existence in any district at the time of enactment of this chapter shall, at the expiration of a period of five years from such date, become a prohibited and unlawful use and shall be discontinued. Lawfully existing signs accessory to a nonconforming use or building shall not be subject to this provision.