I haven't had this problem in real estate yet (and from what I hear everyone has a horror story so it's kind of part of the business, unfortunately) but I have had it with my other business. I'm an SEO and online marketing expert for real estate professionals and I had a web designer that basically owed me $20K in services.
Throughout the whole process, I was careful to make it seem like if he did any of the things below that I would undo whatever I did. However, it was never my intention to undo anything. I just didn't want to make it possible for him to sue me.
First, I wrote a negative review about him on a very popular niche-related site (like BiggerPockets in your case; just be careful if they have a common name so that you don't hurt someone unrelated) and his business using this title to make sure that it ranked in the top 10 any time somebody searched for him or his business:
"Joe Blow of Joe Blow's X in San Diego, CA: An Honest Review After He Didn't Finish the Job AREA CODE + PHONE NUMBER"
I was careful to make sure that I just stuck to the facts and didn't call him names or use words like "scam" or "ripped off." It was basically, "I paid him to do X, Y, and Z and he took my money but didn't do Y and Z." I didn't want to give him the chance to sue me for slander or something.
As an SEO expert, I'm telling you that a post of 150+ words with that Title, including city, state and phone number, will get ranked on page one of Google pretty fast so that any time somebody searches for "Joe Blow review" or something similar, it will probably be on page one.
I waited a few days and made sure that it was ranking on the first page of Google.
Next, I wrote him a letter covering everything below and FedEx'd it because EVERYONE opens a letter from FedEx.
In it, I told him to search for "Joe Blow review". I was careful to make it seem like I would take the review down if he refunded my money BUT I didn't promise it because I wanted to leave it up so that everyone to know about what he did.
Regarding the first-page ranking, I sent him a link to an article explaining that 94% of customers research a business before they hire and I asked him how many customers he would lose because they saw that negative review.
Next, I told him that I had spoken with a really well-known law firm in San Diego. (I didn't technically lie because I did call the law firm and found out that they wouldn't take on cases with less than a $100K lawsuit.)
Then I went to the California State Attorney's website and threatened to report him. And I was really careful to not make it part of a deal that I wouldn't report him if he did pay because I wanted to report him.
Next, I shot a screencast video with Jing in which I told him that I would set up a Google Search alert. I "promised" that I would immediately respond negatively to everything posted about him and his business.
After about three negative responses that I emailed to him, he called me and said that he only had $18K left in his bank account and he asked if I would take $15K.
I was prepared to take the $18K (I couldn't care less if the A-hole was left broke) if he actually didn't have the $20K but my gut said, "He's full of ****. This guy has the $20K." So I pressed him and said, "No, I really want every penny back and I want a cashier's check overnighted to me or I will do what I said I would do. And every day longer I'm tacking on 5%."
I got a cashier's check the next morning for $20K.
Now, I did still give him negative reviews and I did report him to the California State Attorney.
I think you could try something similar and point out that if you have to take him to court, he's going to end up paying attorney's fees for both him, you and a whole lot more... maybe even go to prison for embezzling.
Finally, don't forget about the "time cost of money" and figure out what it's worth to you to get most of your money back quickly. If he owes you $50K, it might be worth taking $40K to be done with it. Sure it sucks to get taken like this but it might be worth the aggravation.
BTW, all of this took me a little over an hour. I think writing this post probably took longer. :-)
I hope it works out for you.