Tenant Not locking outside door on Duplex

[Jennifer Sleider]

I have found that one of the unit in my duplex has tenants that frequently leave the outside door unlocked. This door accessed the shared basement so it could open me up to potential loss as well as heated disputes if stuff gets stolen. 

Any tips? Not sure what I can do here besides ask that they lock it, which I have done multiple times and they said they would.