How to address this pest control issue?

[Michaela Manco]

A month ago one of my tenants in a 3 unit building notified me that they heard a mouse running through their walls and that they believed they were allergic to the dander and had to basically live on Benadryl. They told me they were trying to fix the problem on their own but having no luck and still hearing them in the walls every night. This was late in a Saturday night that I was notified, I immediately contacted pest control and had pest control out early Monday morning. 

Upon letting pest control into their apartment that Monday, I discovered that their unit was not kept in sanitary conditions. They have 2 pet rabbits (the lease allows small pets), they had a bag of rabbit food tipped over and spilled in the cages, and hay outside the rabbit cages in an open bag and also sprawled out on the floor of their kitchen. Other rooms were cluttered and wall to wall clothing on the floor. All 3 units were treated by the pest control company. I informed the tenants at this time that everyone in the building needed to work together to ensure this wasn’t an ongoing issue, that pest control came in to do their job and now it’s on the residents to do their part to make sure the problem goes away. This meant making sure all pet food was stored in airtight sealed containers, keeping hay in containers, being diligent about not leaving food out and taking our trash timely, and also making sure a side door that was often left ajar was tightly closed to not allow an access point for pets. This all seemed well received. I followed up a week later to check and ask if they were able to get bins for the pet stuff and to ask if the noticed any reduction in mouse noise. They said they had noticed a huge reduction, told me they bought some bins and had a few more to buy. 

A month later a neighboring tenant reports a rancid smell in their unit. It turns out this is a dead mouse in the wall. I purchased an air purifier for the unit to rid the smell and had pest control back out to the whole building the same day the smell was reported. Upon inspection of the apartment that initially reported the mouse issue, the apartment was in the same condition it was a month ago. Still pet food and hay out, and no bins that they told me they purchased in sight. Now they are on a month to month and the lease mentions your standard clauses about cleanliness and conditions of the apartment. How would you proceed?