Will 300 watts of electricity ruin a 288Watt LED light bar
If the light bar is designed to run on 12V of electricity, you won't harm anything by running it at 12V (if it's designed for use in a car, it will run on 13.8V without harm, since that's the nominal voltage of the car's electrical system when the car is running - most devices designed for automotive use will run fine from around 12V to around 15V).

The 10 - 30 VDC rating if your Lightbar means it will operate normally at any voltage from 10V - 30V, and it will only use as much current as it needs, it won't suck up all 30A of current from your power supply. Another way to look at it: a car battery is capable of putting out hundreds of amps of peak current, so if the device worked the way you think it does, as soon as you hook it up in a car, it would suck down as much current as the wiring and battery would allow -- possibly thousands of watts.

If you were building your own light bar out of discrete LED's (or something like a an LED light strip) rather than using this device that's already designed for 12V use, then you would need to be concerned with current draw. LED's are "constant current" devices, they normally need a special LED driver that will limit the current to the LED to the correct range and if you hooked a typical LED to your 12V power supply, it would try to use as much current as your power supply can provide until something burns up (most likely, the LED would). 

But in your case since your Lightbar is rated for automotive use, you can safely plug it into a 12V power supply without worry.