The process is difficult for some students to see. When the rubbed balloon is brought near a metal, the positive charges on the neutral metal are attracted to the near side and the negative charges are repelled to the far side. Polarization is a process by which a charge can be effected on the surface of an insulator. A rubbed balloon sticks to a wall by the process of polarization of the molecules in the material of the wall. Many molecules have one end that is slightly more positive and the other end slightly more negative.
Water is a good example of such a polar molecule.
Normally, the molecules all point in random directions so there cannot be any NET effect of excess charge on any surface. Nothing has been permanently changed in the material that is near to the charged object so when the balloon is removed, the molecules revert to their original jumbled state, all pointing in random directions, so the polarization effect is not permanent.