Please go to Setup> Settings> DLP/Compliance Settings, and under the 'Encryption App Settings' section, you will see the domains listed. Here, you can switch ON/OFF encryption at the domain-level. [refer snap1]

You can also switch ON/OFF encryption at the user-level by clicking on the '>' beside the domain name which reveals the list of users in the domain. You can search for a particular user and choose to switch ON/OFF encryption for that particular user. [refer snap2]