I've read a vinegar or citrus perimeter barrier can work.

What I did, and it worked perfectly, was I got a gallon of white vinegar, baking soda, hydrogen peroxide, and mixed some of that in a 5 gallon bucket.

I think being a woman could be an advantage for you since I think people perceive women as inherently more trustworthy.As long as you don't treat people poorly, they will often be happy to talk to you and help you.I've seen property buyers come in and talk down to people, and that's a recipe for failure.Remember, sugar instead of vinegar...

How cute.They say you can catch more flies with honey than vinegar but I don't think that works with tenants.The only incentive I use is the tried and true method.

One way to find out if that ductwork is also going to your neighbor's apartment might be to close your vents, turn off the heat and turn on the fan, and place a stink bomb (egg and vinegar, see http://www.wikihow.com/Make-an-Assortment-of-Stink-Bombs) in the exhaust draft.

I instructed the tenant to take down all the curtains and wash in hot water with vinegar.

Just for fact finding (and you probably, well always, get more flies with honey than vinegar, so be nice)...

Work out more, I got a lot less flak from everyone once I hit 190lbs of piss and vinegar.  

If a porous surface, clean with a vinegar mixture (half water/half vinegar). 

Then bought the Simple Solution, but couldn't tell if it needed to be watered down or sprayed directly or what (it was a gallon jug), so I called them and they said dog pee, specifically, dries to an alkali, and to bring the Ph level back closer to neutral would help the enzyme work better, so they advised to spray a white vinegar/water 50/50 solution on the pee spots and then let it dry.