Limiting amplifier output power
Output power will be around 2W. However, I need 1.5W as output power.


I presume from your question that you have a 1.5 W loudspeaker and you are worried that a 2 W amplifier may damage it.

You don't need to do anything except turn down the volume on the audio input.
You will hear distortion (a harsh sound) from your loudspeaker before it gets damaged. When you hear distortion you know that you need to turn the signal volume down.