So far I've only communicated via text or face-to-face that they are violating their lease, but they are still doing so and just trying to hide the smell. 

There is sort of a East side/West side thing when it comes to investors that almost sounds like the rap battles of the 80's. 

It's not about losing the battle, it's about winning the war.

Not worth the legal battle.

I know cat urine can be extremely tough to get rid of and will have to possibly sand and deep clean floors and then seal them.Main question: Is it possible to fully remove the smell of cat urine?

Just not passing the smell check.

So it could end up being a drawn out legal battle that I wouldn't want to be involved in as a renter living there personally. 

One of my LTR tenants is a heavy marijuana smoker and the smell is invading my Airbnb.

So you are fighting an uphill battle.  

There are all kinds of scams and bad deals , out there and they can smell a newbie a mile away .