How do I set my Canon digital SLR to name its files differently (with a different prefix)?
It would be convenient if you could set the file prefix, yourself, but it can’t be done. The prefix is set in the factory, in the hardware, and each individual camera has a unique prefix to identify which camera produced the file. If you want to use a Canon digital SLR, the unwanted file prefix is an annoyance you will just have to get used to.