Reduce audio amplifier output power
Don't bother.  2.5 W is unlikely to hurt a 2 W speaker.  It will just cause distortion.  Somewhere in your audio chain there must surely be a volume control.  Turn down the volume to the point where the sound from the speaker isn't distorted.

The 2.5 W rating of a power amp is what it can put out, given the right signal and load.  Put in lower volume, and you get lower volume out, which also means less power out.