What stops me from reformatting a DVD+R?

Put in fairly simple terms (and how I understand it, I could be slightly wrong about the actual manufactured materials, but I believe the process is as follows):

• Pre-recorded discs have small holes in the surface that will prevent the reading laser from being reflected, giving you a reading of a 0 or a 1.
• Recordable discs have a dye that can be burned through by a drives writing laser. The gaps in the dye now work the same way as a pre-recorded disc would, representing a 0 or a 1 based on whether it is reflected back or not. Once this dye has been burned through, it cannot physically be re-recorded (although I guess you could burn out the entire surface but not make anything useful).
• Rewritable discs use a type of metal surface instead of a dye, that can be changed by the write laser, depending on the power of the laser used on it. This makes the metal layer reflect differently where the laser has been, and can be "reset" by a different powered laser.

As such, a writable disc is permanently "set" in the way it is by a write laser, with no way to reset the damage it does to the dye in order to write the data.

Further reading: All about CD-R and CD-RW (albeit related to CD-R/RW technology)