Delete or Remove Site Collection Administrator

1. Open your SharePoint site.
2. Select Site contents.

3. Select Site settings.

4. Select Site collection administrators.

5. Here you can remove users from the list of Site Collection Administrators by selecting the x icon to the right of the user.

Or you can run a powershell script
"You will need to be a SharePoint or Global Administrator to run this script"




Import-Module Microsoft.Online.Sharepoint.PowerShell -DisableNameChecking




#Variables for processing




$AdminURL = "https://crescent-admin.sharepoint.com/"




$AdminAccount="salaudeen@crescent.com"




#Connect to SharePoint Online




Connect-SPOService -url $AdminURL -credential (Get-Credential)




#Get All Site Collections




$Sites = Get-SPOSite -Limit ALL




#Loop through each site and remove site collection admin




Foreach ($Site in $Sites)




{




    Write-host "Scanning site:"$Site.Url -f Yellow




    #Get All Site Collection Administrators




   $Admins = Get-SPOUser -Site $site.Url | Where {$_.IsSiteAdmin -eq $true}




  #Iterate through each admin

    Foreach($Admin in $Admins)




    {




        #Check if the Admin Name matches




        If($Admin.LoginName -eq $AdminAccount)




        {




            #Remove Site collection Administrator




            Write-host "Removing Site Collection Admin from:"$Site.URL -f Green




            Set-SPOUser -site $Site -LoginName $AdminAccount -IsSiteCollectionAdmin $False




        }




    }




}