Put a lightbulb in line with the speaker. Yeah it sounds stupid, but it works - as the volume increases (ie the power output increases), the voltage on the speaker line increases. As the voltage increases, the light bulb begins to glow, and the tungsten filament, as it glows, increases its resistance.
All in all, it's a really cheap speaker protector. It used to be found all the time in older '60's speaker bins. Now polyswitches are used, but they are on/off type things.
Don't know just how much the lightbulb reduices the signal though - tryingto drop a flat out 800W down to 200W might be a bit hopeful.
I've used this to limit speakers that weren't too far behind the amp - 150W speakers and 200W amps.
- gaffa