Nope. You can always reformat your USB again and fill it with
what ever you like.
Unfortunately that's not my experience. It seems that most flashdrives (including Kingston) can be made bootable once, and they will work OK, but the problem comes when you try to reformat them ready for burning a new .iso on to them. I've destroyed two flashdrives this way; something (but I'm not sure what) happens to their boot sector and they become read-only and can't be reformatted. I've tried a whole host of different tools to try and resurrect them without success.
I've posted some details of all this at this tomshardware page. It seems that the boot sectors on different flashdrives are all different and proprietary, and they react in different ways to having an .iso burned on them. Some people have reported they've been able to successfully reformat and re-burn new .iso's on to their flashdrives, so I know it's possible to do this with some brands, but I need to know which ones. Before anyone burns an .iso on to a flashdrive, they should find out what brands allow this process to be recycleable and which ones don't.
If anyone has info on this branding issue, please post it on the above tomshardware url or e-mail me directly on firstname.lastname@example.org. Meanwhile I'm trying to find out which flashdrive brands support making them bootable, and which ones don't. especially since I've discovered the hard way that Kingston don't.