How can I disable MySQL from starting at boot?
Open terminal, then:

sudo launchctl list | grep -i mysql

launchctl remove xxx.xxx.mysql


Where "xxx.xxx" is included in the output of the first command, for example "org.macports.mysql". The password for the "sudo" command is your own user account's password.

True, sudo isn't needed to "list" but because you already gave a sudo password for the first command, it is stored for a short while in session memory and isn't required for the second command (which is privileged). Otherwise, just skip sudo on the first command and use it on the second. There are more ways than one to skin a cat.