Even if it isn't required in your area I would suggest and Implement a Crime Free/ Drug Free
Addendum.. It highlights , suspicious activity, High Traffic, as a Zero Tolerance lease violation.
If it were me I'd give them a mutual lease termination notice,, give them a week to move, return the deposit, if apartment is left broom clean and they are out. If they don't agree to this then let them know you will file for cause, High traffic, ... business based, or not,, it's wrong and not part of I bet anything in your city code as far as running a business out of an apartment, even if it's not in your lease,, city code would be enough to use as reason.
In the mean time,, post parking lot no trespassing unauthorized cars towed,, give out parking stickers for tenants,, no one in the lot that doesn't live there, and watch that lot.
Call police ask for drivebys, any reports from tenants to you have them also call police,
In this business sometimes you have to yourself camp out in the building,, I'm serious,, grab a chair sit by the entry door inside.. and protect your asset,, If they don't belong there don't let them in.. Change the buzzer code,, change the entry door lock key,, so copies aren't out there.
JUST plain make it so uncomfortable for them that you interrupt what they are doing until they leave.