Example from DT -
Unlimited seats (U-EL):
This license extends our regular Royalty Free / Editorial license to an unlimited number of seats within the same organization. It is an additional license to the usage included within the regular Royalty-Free / Editorial license that awards rights for a single person within the same company. The U-EL license is applied only for the staff of the organization that holds the account. The maximum amount limit of 500,000 copies applies for the total amount of work resulted from all designers involved.
There is no way to enforce this or any other license since the agency does not have access to the records of the company who bought the images.