Does money taken for work in designing free wrestling advertisements, which is believed to be unlawful, and then disposed of, remain a debt even if the employer does not demand it and is difficult to contact? And is the person considered a thief in this case?

If you took money from someone and did not perform the work agreed upon, you must return the money to him, unless you are certain that he has waived his right to this money. In this case, he is not permitted to retract the waiver, and you are not obliged to return the money, because "what is waived does not return." However, if you are not certain that he has waived the right, then you must return his money; because the money was definitively established as your liability and is only discharged definitively. If you cannot reach him, then give the money in charity on his behalf.