How do I allow or restrict services per user or per domain in IceWarp?

This can be accomplished by doing the following:

Navigate to Domains & Accounts > Management, select the domain then choose the appropriate user account, click on the "Policies" tab of the account, and check/uncheck the desired service.

You can also do this on a domain-level.

Make sure you click the Save button. Once this is done, the specific domains or users will not be able to receive the service you just restricted.