You should be issuing a immediate legal notice for rent owed. You would then go through the process to collect.
Legally a landlord may not use a security deposit to cover last months rent unless your state landlord tenant regulations state otherwise.
You need to study/learn your state codes. Learn the laws
What @Thomas S. said, plus the fact that if you let them use security for last months, you have nothing for property damage. It's security for property, not rent. I think we could all do better to educate tenants to the fact that in today's digital world, they can't get away with this crap anymore. They need to know you will file for eviction if they do not pay their last month's rent and only an incompetent landlord will fail to conduct an eviction search on an applicant.
@Robert T Boll At this point, since the 15th is just a few days away, it's clear your tenants are hoping to skip out without paying you the last months. Despite what was said above, you can use the security deposit to cover any unpaid rent the tenant still owes when they leave. It's not advisable to let them do this because then there will be nothing left over to cover damages that are left (and that's why leases usually prohibit it), but tenants often do it anyway and when they do you can legally retain their security deposit to help offset the money you're owed for rent.
I wouldn't tell them you're going to do it though. Here's why....if you decide to end up pursuing them for even more money for damages, you'll have to do that through small claims court. And to do that, it'll be helpful to know where they're moving to so you can serve them with paperwork. So if it were me, what I would do is when you collect the keys just ask them for a forwarding address where to mail any balance of their security deposit that may be due to them. You don't have to tell them at that point that one isn't likely to be due to them. However, you'll need their address anyway to send an accounting of security deposit deductions, which is required even if you're not returning any of their deposit.
They may know they're not getting any of their security deposit back and refuse to provide a forwarding address to you, in which case you still have to mail the accounting statement to their last known address (which would be your rental property).
Ultimately, whether or not you decide to take them to small claims court will depend on many factors that only you can evaluate, such as how much money is owed, whether or not it's worth your time to try and recover that money, the likelihood of being able to collect on a judgment if you are successful in court, etc.
I would have issues a notice to quit when they didn’t pay the rent. Maybe they aren’t planing to leave on the 15th. There are no guarantees.