We have purchased 20 licenses for Citrix Access Essentials (CAE) and there are 30 potential users and only will be 15 concurrent.
The way to not purchase more licenses is to manually revoke licenses through the Administration tool.
There is any way to revoke all licenses through command line or any other automatic way?
According to the "Licenses: Migrating, Upgrading, and Renaming" documentation on the Citrix website, it looks like you could do manual deletions of the .lic files, but it's not recommended because if the metadata isn't updated properly it would screw up Citrix's license counts (see page 11 of linked PDF).
The "Access Essentials Administrator's Guide" only mentions one way to remove licenses (one-by-one, per user from the Admin tool, like you mentioned) so unless you want to mess around with the .lic files, I think you're stuck. Sorry.