Tenants leaving early. Not paying last months rent. BEDBUGS?

[Lue Brooks]

I recently have experienced my my first move out. The tenants left a month early. I told them that they are still responsible for the last months rent but I would find a new tenant to replace their time to help them save on moving out early. Well they ended up leaving and taking the keys with them. I got in and have inspected it and I saw a bed bug spray in there in the lease it said they are responsible for bed bugs. I called to get them to give me their return address but they ignored my calls but I found their current mailing address on google. Am I able to mail them a bill for the remaining rent plus a bed bug inspection? Also the security deposit can’t be used toward rent.