We have problems in alleys here in Phoenix. A cheap camera pointed at the alley (high, out of reach) might help. A motion detector triggering a flood lamp may also deter some. If it is not a residential area, streaming of Bach or Chopin music ruins the mood for druggies. If classical is not your style, you can go with Barry Manilow, Bread, The Association and the Carpenters.
They tried that at 14th and Broadway in downtown Oakland and the druggies still stuck around.
Buxtehude on the grand organ, or Lord Nelson Mass by Haydn, or "I Got You, Babe" by Sonny & Cher.