I haven't tried this by myself with spi nor, but it could unbrick your board. There is rockchip devtool. Get it. Install it on a linux host, which you will attach rock64 to. another SBC for example. You need to connect rock64 through usb-usb to the host, without SD and eMMC, holding some button when powering it on. I haven't done this with rock64, but I did so with a rk3368 settopbox. it's probably a reset button. you hold it and while holding, power the board on. for the host it's seen as a otg client. then use rkdevtool utility on the host. it kinda can (should be able to) erase the flash. read the commands on the page in the link.
ANT - my hobby OS for x86 and ARM.