This sounds like a scare tactic and their trying to extort money from you. 

As both a short and long term rental owner I’ll just add a few words of caution here...AirBnB and other vacation rental sites like HomeAway are great for the purpose they originated: short term rentals, but they offer minimal protection for the concerns of many owners needing to secure their properties for longer lease terms.As a traditional property manager I’m far more interested in ensuring I pull a full criminal background check, speak with a potential tenant’s present and prior landlord, confirm consistent employment/income sources for > 6 months, and ensure a potential tenant has a track record of paying other bills they have agreed to in writing like credit cards, car loans, and others.On the contrary, at best AirBnB/HomeAway/others give me the following info on guest/tenant: a blurry selfie (that could be their dog)confirmation of a non-specific government IDconfirmation of an email address I can’t seecurrent city/state of residencelist of reviews by individuals running a different AirBnB (that I have no way of confirming is not the guest’s best friend)Property insurance secured by (and in the interest of) someone who is not meWhile I’m OK with taking a gamble on unknown guests using my property for short term stays, stays of longer than 1 month generally involve: Someone looking to move significant belongings in with them (that I cannot legally remove in most states)Moving in for a period of time they’re generally unable to pay for entirely upfrontTenants receiving access only after a legally-executed lease, criminal, background, credit, and income check that’s par for the course in a professional, term stay arrangementEven beyond the blatant lack of risk mitigation tactics common in long term lease agreements, the fact remains that many STR platforms are also skimming 10-20% of gross revenue off the top of owners’ profits for little beyond the initial SEO and payment processing.

I’ve had success with this tactic as well as informing them I will hire a collections agency, garnish their wages etc.

When I first heard this tactic a few years back I thought it sounded kind of sharp, but after I had thought on it for a while I decided that if I'm too skittish/nervous/scared/shy to tell the tenant that it's my property I had better use a real property manager and move on to other issues.

Wow, very judgemental bunch.They are kind of right and generally you should use first come, first serve to eliminate any possible discrimination lawsuit...but, I applaud your instincts and tactics to keep looking until you find the absolute best tenants, but keep yourself protected by clever deposit agreements.

You can do this by installing a floating floor in the upper unit or using a 2nd drywall ceiling in the basement isolated with j-clips. 4) Other secondary tactics involve installing material that absorbs sound energy.

It's by far the cheapest option.And odds are, that if/when they go get advice on how much they can sue you for, the lawyer/fair housing people will tell them they should be glad of what you've done for them, and they could owe you money if they sue you, so they should leave well enough alone.And, as I said, I'd tell the manager to forget about getting any more money from you, and if they disagree, they're welcome to sue you and you'll counter-sue them, as well.I've had excellent experiences using this tactic.  

However I will definitely being employing that tactic down the line.

As a practical matter if the tenant has any tactical sense (and it sure sounds like they may) they could probably challanege a scrivenor's error defense by claiming ignorance of the landlord's "big business" thought process:  "I thought maybe he had some reason for it, my friends suggested maybe it was tax issues, or insurance or that kind of thing or maybe he was doing something illegal and I would be in trouble if I said anything.

This was a scare tactic to get us to settle.