Charlie Sheen has been texting several porn stars since he began treatment in his rehab program -- but TMZ has learned his messages are clear ... the party is OVER.

Among the messages Charlie has sent -- "Please lose the number, we are closed ... please drive through ... thank you."

Another text reads, "Right now we are on lockdown."

We're told Charlie is taking his treatment seriously and wants to go back to work by the end of the month.