I don't know about your particular pump, but I have worked on many pumps with impellers. Most likely, the impeller is either threaded to the motor shaft and will have to be removed by turning in the same direction as rotation. When running the impeller is continuously tightened. I have accomplished this most easily by lightly tapping with a blunt punch in a thick part of the impeller if the motor cannot be secured from turning. Care must be taken to avoid bending the shaft while doing this. The impeller could also be "keyed" to the shaft and secured with a set screw.