The easiest way is to get something like Paint Shop Pro 7 (8 is awful). Using either the Selection tool or free hand selection, surround the characters and use Gaussian blur to blur out the plate.
There are some other free applications out there that can do it for you as well.
As mentioned, you can't stop the normal person on the street taking your plate number down but the Internet is a resource for all and I'd rather my plate not be available to view on the web.. as mentioned, it's just one preventative measure
