Hi,
usually user dietpi should still have access as you lockout user root only. Probably something wrong on your Dropbear server. Do you still have local access to the system? Did you tried to reboot.
I did a small test and added -w option
# any additional arguments for Dropbear
DROPBEAR_EXTRA_ARGS=-w
once Dropbear was started, root was not able to login but user dietpi is still working