1) put a valve in the output line and use this to regulate the flow. Restricting the flow is just like asking the pump to pump the liquid higher. It'll only pump so high, and output is reduced the higher the hose end is.
2) never use a dimmer with a motor. There is something that looks like a dimmer, a motor speed control, as used for overhead fans, that many motors are compatible with (e.g. my Teel magnetic drive pump I use for my rims circulation). Get them in your local Home Depot where they sell lighting and overhead fans. Your pump motor? Who knows. If it has a thermal cut out, I'd be more inclined to try it, since it should cut out before damaging the motor.