If you try to change the service access for a particular user/domain but found the service was grayed out, like so -

you need to change the Access Mode option for the service.

Each service has its own Access Mode setting. This example shows how to set it for the Antivirus Scanning service but it is similar for the other Services.

Go to Anti-Virus menu node.

Click the Access Mode button, a new dialog appears -

The default setting is All Accounts.

If you need a more refined control of service access (e.g. based on Domains or Users) then you need to select the appropriate option.

For more information please see the Administration Guide.