So your saying that city lawyers are too stupid to put a lien on someone's property?

If you own anyone money that person can take you to court and seek a judgement. If that judgement is granted then that person can put a lien on your property. The fact that the property is outside the jurisdiction of the city is irrelevant. They could file the lawsuit in county or state court.
This is an example of how in someways half-baked libertarianism is worse than statism. In a truly voluntary society there wouldn't be a monopoly on fire service. Multiple companies (none backed by any government) would compete to provide the service. Some would refuse service to anyone who didn't prepay. Some would take future payment and secure that payment with a property lien. Some would do a mix. The other place were half-baked libertarianism is worse that statism is the current incarnation of "private prisons". Companies like CCR lobby for more laws with stiffer penalties because they have a vested interest in locking people up. There are libertarian ways to de-tangle both of these problems, but the answer is not to declare a feces-sandwich to be a quarter-pounder with cheese just because it has the look and feel of "smaller government".