To do this for dropbear is quite simple, on Debian/Ubuntu based systems:
Head to /etc/default/dropbear
You may have a quite a few lines or you may have little, this just depends on how it was installed.
# disabled because OpenSSH is installed
# change to NO_START=0 to enable Dropbear
# the TCP port that Dropbear listens on
# any additional arguments for Dropbear
As seen above, you will want to have DROPBEAR_PORT=51096 on a line to change the default dropbear port.
To disable root password login, root logins, etc. you will need to have DROPBEAR_EXTRA_ARGS=xxxxx on another line.
-g = Disable password logins for root
-s = Disable password logins
-w = Disable root logins
An example would be DROPBEAR_EXTRA_ARGS=-s
to disable password logins on all accounts including root.
Finally you will want to service dropbear restart
restart to finalize the changes.
This is taken from my other post. Just thought it would help some others.