I'd just tell them "As you know, this is a non-smoking apartment, and the code is strictly enforced, I don't want you all to waste your money if it will be an issue." --  this sets the tone that no smoking is allowed, and makes you seem nice because you care about them not throwing their money away.   

If in fact they always ask around when rent is due, and then use that as an excuse to late pay, don't pay, and/or partially pay rent, then yes I would get their behind out of there so fast it would make their head spin, and I'd do so by legally enforcing the contract.On the other hand, I have had tenants that were just really picky ... the first few months they asked for a bunch of repairs.

I'd contact law enforcement at this point.

If they are VERY professional, they will have their processes in writing as verification that it is enforced equally and fairly by their entire staff.6.

This has also helped me gain a lot of knowledge about different building departments, how they interpret and enforce codes, and the specific things they want to see on their drawings in order to expedite permit approval. 

DO NOT LEAVE for the benefit of your Aunt until you are escorted out by law enforcement.

It doesn’t mean that John Doe is the party to a legal enforceable contract.I can create today a contract saying that Charlie MacPherson owes me 10 Millions dollar in exchange for a hot market tip that I gave him.

They hold all the original docs and do the monthly e-payments and amort, annual interest reporting, enforcing late payment fee if any,  etc.   

Every situation is different, and every agreement has different terms, so there are lots of things to consider in order to determine if the contact is valid, legally binding and enforceable.

If they are VERY professional, they will have their processes in writing as verification that policies are enforced equally and fairly by their entire staff.6.