Prevent/Restrict users access to the 0365 portal?

hi philcrombie,

it seems that you want to restrict your users from using outlook web app but only want to allow them use outlook client.

if this is the scenario, please follow the steps below to achieve it:
1. login to exchange admin center (https://outlook.office365.com/ecp/) with an admin account.
2. navigate to recipients->mailboxes and double-click the user account.
3. in the pop-up window, select mailbox features and click disable under email connectivity.
    

if anything is not clear, or there is any misunderstanding, just feel free to post back. we’re always here to assist you.

thanks,
richard

Hi Phil,

Yes, there is. To quickly disable OWA for a mailbox by using PowerShell commands, you can:
1. Connect to Exchange Online using PowerShell.
2. Run the command: Set-CASMailbox -Identity "MailboxName" -OWAEnabled $false
For more information, please refer to:
https://technet.microsoft.com/en-us/library/Bb124124(v=EXCHG.150).aspx

As a tip, if you want to disable OWA for multiple users at a time using PowerShell, some PowerShell script should be involved to achieve the goal. As our forum mainly focuses on basic PowerShell command related issues and question, to ensure you receive the best support, it’s recommended you post this question at our PowerShell script forum for professional support.

Your time and understanding are highly appreciated.
Mouran