Get some maniquins and dress them up like cops, security officers, and move them around. Fake Cameras high up but still visible because of the little red light inside the box, specifically visible so they can see there is a "Camera" or 2. And put a sign up with it printed clearly that this area is being monitored 24 hours a day by your local law enforcement.

...Putting lights in an alley is just something to throw rocks at.

...So you got to make your camera box look real enough...a soda bottle bottom end looks like a camera lens. Paint it black and remember to run a wire from it to a telephone pole high up and just tack the wire to the pole if it is wood. A metal hose clamp that goes around the pole and a square wad of metal looking stuff is visible and clamped behind it and it looks like some kind of wireless transmitter.

Or a light that senses motion and turns on but in your case not a light but a wailing noise loud enough to wake up the neighbors. You got to look for this stuff.

Lights.... and lots of them. If you want to deter questionable people and questionable traffic then you light the place up like a christmas tree and make it uncomfortable to do questionable business in the alley.

Motion activated lights; they need to turn on and off quickly and turn up the sensitivity. The indecisive nature of the light will deter them.