If the 'Virus Scanning' feature is enabled for a cPanel user, but the cPanel -> Virus Scanner interface shows "ClamAV scanner has been disabled for this account.", this could be because no ClamAV scanning options are enabled for the user.
First, ensure that the 'Virus Scanning' feature is enabled on the user's feature list. Check WHM -> Feature Manager -> Select the user's feature list from the drop-down menu -> Edit to confirm this. The 'Virus Scanning' feature should be checked.
After confirming that the Virus Scanning feature is enabled on the user's account, ensure that the user has options configured in WHM -> Configure ClamAV Scanner:
1) Open WHM -> Plugins -> Configure ClamAV Scanner
2) Click 'User Configuration' under 'Configure ClamAV Scanner for individual users:'
3) In the 'Configure User:' text box, input the cPanel username
4) Check the boxes next to the options that you would like enabled for the user
5) Click 'Save Defaults'
Returning to the user's cPanel -> Virus Scanner interface should now show the same options that were just enabled in WHM -> Configure ClamAV Scanner.