For Google Workspace.

Question:

How to restrict access for mail clients that use POP/IMAP/Sync?

In order to restrict the access to POP/IMAP from a mail client or to restrict the access to Google Sync from a smart device, set up the setting on the Google Workspace Admin Console. 

Please refer to the Google Workspace Support site shown below.

Settings for IMAP and POP access

　https://support.google.com/a/answer/105694?hl=en

Google Workspace mobile management /settings

　https://support.google.com/a/answer/1753793?hl=en&ref_topic=173419

This settings is the Google Workspace feature.
For further details, please contact the Google Workspace.