Tenant wants to use last months rent to pay past due rent

I rent a house to some tenants in Jacksonville, FL, who have rented from me for over a year. At the lease signing I asked for and they paid first months and last months rent and a security deposit. The lease they signed specifies that the sec. deposit can't be used to pay last months rent, but there is no other mention of last month's rent in the written lease. They are now behind on their rent and want to use the last month's rent they paid at the lease signing over a year ago for the rent they are behind on now. Am I obligated to apply the "last months rent" they paid at lease signing to their current rent that they're behind on (something I don't want to do), or is this something that's at my discretion?