Can a power amp have too much power, risking damage to a speaker?

NO!

Too much power generally isn’t an issue when pairing hi-fi /home theater speakers and amplifiers. It’s the opposite situation—too little power—that can create problems. When an amplifier is driven into clipping (distortion) by being asked to produce more power than it’s capable of delivering, the voice coil of the speaker it’s attached to can overheat and become damaged. That said, an RMS power rating on a speaker doesn’t even begin to tell the whole story. Two other specs, sensitivity and impedance, are much more important in determining how much power a particular speaker requires to deliver sufficient volume levels.

Ya, too much gain.

Rock "in" Roll

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You'll likely damage your hearing before the speakers 

Technically yes, you could damage your speakers with over wattage, but as someone stated, your ears would probably be damaged first. There is more risk with an underpowered amp that starts clipping at high volumes, this will trash your speakers much faster. 

THANK ALL of you for usefully responding to my query.
My overall take-home: an underpowered amp driven to clipping seems more likely to damage a speaker than clean power that controls the linear motor of audio transducers. An amp driven at its upper limit is more likely to clip tops of waves, causing the voice coils to stay in one position just drawing current [may not have said it perfectly]. 
I still have great hearing even after being in a rock band during my 20s [took great care to protect ears during all performances], and we often get out an audiometer to ensure the sound-pressure-level is well under 95dBA for sustained periods. So far, so good!