Do not turn off the water.
You can be held liable for three times the rent for the length of time that the water (or any utility that you are responsible for) is off.
Did you ever accept rent from the people who are living in the house now?
Do you know their names?
Did you ever get the keys or a letter turning over possession to you or your agent from the original named tenants? Do you have proof that the original named tenants abandoned the property and took up residence elsewhere? If not, then technically, they may claim that they are still in possession. They may show up a week from now and claim that never intended to move out.
You may need to sue the original named tenants for possession using the three day notice or you may need to sue for unlawful detainer or ejectment of "unknown occupants."
Please go see a local real estate or landlord/tenant attorney. Good luck!
You cannot use self help remedies such as turning off the utilities or changing the locks on the door. To legally remove them from your property, you must take action through the courts. You should contact an experienced real estate lawyer in your area about bringing an action for unlawful detainer or an action for ejectment.
Disclaimer: This answer is provided for informational purposes only, does not constitute legal advice, and does not create an attorney-client relationship. Actual legal advice can only be provided after completing a comprehensive consultation in which all of the relevant facts are discussed and reviewed.
My fellow attorneys are giving you the straight answer. You need to resort to court action to remove theses people. You can get the forms from the Courthouse for I believe a small fee or online for free (I believe Pasco has the forms online- Hillsborough has them for sure). You need to evict these new persons and the original tenants unless you can determine they abandoned the property, which I would bet against. I would get a forwarding address of the original tenants and send them a certified letter telling them you are keeping the deposit because of unpaid past rent they owe you. Hopefully that letter is sent within 30 days of their abandonment. If you have no forwarding address, send it to the last known address - the house- they may have a post office forward order placed and if not, you did the best you could. And you can evict someone not on a lease. You probably need a lawyers help because these issues could be tricky and frustrating. Use a Tampa Bay local attorney. It might you less than you think.
Michael Lygnos, Esq.
Lygnos Law Firm
Legal disclaimer: This answer does not constitute, legal advice nor does it constitute an attorney-client relationship. www.lygnoslawfirm.com Free Consultation 813 259-9494 Michael Lygnos, Esq., Lygnos Law Firm P.A., Serving Tampa Bay, Florida