Update:
It's a man and his wife who is pregnant. She has all sorts of time on her hands and has been doing the majority of the flagging/emailing. Her general disposition leans toward being insane/strange , based on the chest tattoo, pink hair at 30 as a business owner, and childish emails.
Today, I as I met with my client as I was pulling out, she took a picture of my license plate and car, presumably to find out my information so she can stalk/harass me as well. I decided I would have none of that. I walked past his office slowly peeking in making sure I got his attention (his wife was nowhere in sight). He followed me shortly back to my clients office and questioned why I walked by. I replied calmly "I wanted to inquire why your wife was taking pictures of my car, that's a tad suspicious". He shrugged and said he had no idea and left. I then got a C&D written up with two copies, one for our file, and one for the owner of the property management company that leases the spaces that my client and now his competitor occupy. (Did I mention they decided to move into his building?)
I walked into the owner's office and delivered an Ari Gold/Jordan Belfort worthy oration on the annoyance of dealing with childish antics while trying to run my business as well as ensure the success of my client's. At first he was skeptical then his apathy turned to worry at having issues under his roof. I told him the letter was a statement or a message rather that if they want to play games we don't have the time and we will ensure they won't be enjoying them if they want to play with adult's time. He shook my hand and asked me to bring any other issues or problems to him immediately before I take the legal route further. Now, I have him as a referee of sorts if this bullshit gets out of hand/escalates.
I slide the C&D in an envelope under their door.
As I am standing outside mulling over my next move and their possible response, the wife walks past me and I follow her with my gaze inside trying to read her, nothing, stone faced staring straight ahead. As I watch her open the door I see the husband standing just on the inside glaring at me. I laugh. The ball's in their court now.