Defects in the form of cylindrical amorphous tracks were introduced into YBa2Cu3O7 crystals by irradiation with 5.3-GeV Pb ions. The columnar defects formed provide maximum possible pinning of flux lines parallel to the tracks and induce giant enhancement of magnetic irreversibility. However, even for this strong pinning, magnetic decay was measured at high temperatures. Relaxation is nonlogarithmic and exhibits an increase of the effective barrier for flux creep with decreasing persistent current. This behavior is interpreted in the framework of the nucleation creep model.