coreskills.yml ships in the mcMMO config folder with only two entries, and just one of them does anything. Default values can be viewed on GitHub.
Changes require a full server restart.
The only key that changes anything is Acrobatics.Enabled. Setting it to false stops the Roll sub-skill from being registered:
Acrobatics:
Enabled: false
The only other shipped entry, Acrobatics.Roll.Enabled, is not read by the code, and the same is true of any other skill or sub-skill Enabled entries you add.
To restrict which skills or sub-skills players can use, use permissions instead. See Permissions.