Seeing how being an admin (not superAdmin) is a fools errand
(after all they cannot actually send an email)
It has occurred to me if you are able to lock him/her down, why is it they can always edit lists?
If you are going to have a permission set that allows or prevents them from:
Manage Subscribers
Send (but not send) Campaigns
View Statistics
Change Settings
There should be one more :
Manage Lists
Personally I do not trust the some admins to not hose up a description or label.
Especially those I will not permit to email.
While I am at it, can you restore the admins permissions set that allowed them to send email.
Using a cron job is a cloogy way to work around not having a permission.