Is it permissible to change a job advertisement for a non-existent position as a joke, with the intention of informing the applicant of the truth later?

The basic rule regarding lying is that it is forbidden, whether for jest or otherwise, and whether the person being lied to knows or not. In addition to the deception it involves, it has been authentically reported from the Prophet, peace and blessings be upon him: "Deceit, trickery, and betrayal are in the Hellfire." Therefore, such an announcement is not permissible.