The AVR Dragon documentation says:
NOTE: When the DWEN fuse is enabled the SPI interface is overridden internally in order for the OCD module to have control over the RESET pin. The debugWIRE OCD is capable of disabling itself temporarily (using the button on the debugging tab in the properties dialog in Atmel Studio), thus releasing control of the RESET line. The SPI interface is then available again (only if the SPIEN fuse is programmed), allowing the DWEN fuse to be un-programmed using the SPI interface.
But I can't find "the button on the debugging tab in the properties dialog in Atmel Studio". My debug menu has "Disable debugWire and close", but it is grayed out. Atmel Studio 6.2.1563, Service Pack 2.