You wouldn't want to degauss it even if there were a button, because it would do no good. In any event, the degauss circuitry is enabled briefly each time you power up. Because your problem is not due to magnetism, demagnetization isn't the answer.

As danamania said, if it's really a piece of dust, then there's not much you can do other than to rap on it. If it really bugs you, go ahead and give the monitor a good slap on the top or sides. I would do this after letting the monitor sit unpowered for a day or two (to discharge any HV that could keep that speck in place through "static cling").

I wish you good luck!