iCloud signed into wrong user on iMac
Just sign out of iCloud and your Apple ID. 


Go to the System Preferences>iCloud>Sign Out
Go to the App Store>Store>Sign Out
Go iTunes>Account>Sign Out


That should log you of everything associating your Apple ID with that user.

For more Info:

Signing out of iCloud: https://support.apple.com/en-us/HT208242

Signing out of iTunes: https://support.apple.com/kb/PH19633?locale=en_US

Signing out of App Store: How do you log out of the Mac App Store?