Scenario: You want to prevent users from configuring forwarding within Outlook on the Web (OWA).

Solution: Run the following in Exchange PowerShell

To Create the Policy:

New-ManagementRole MyBaseOptions-NoForward -Parent MyBaseOptions
Set-ManagementRoleEntry MyBaseOptions-NoForward\Set-Mailbox -RemoveParameter -Parameters DeliverToMailboxAndForward, ForwardingSmtpAddress

Set-ManagementRoleEntry MyBaseOptions-NoForward\New-InboxRule -RemoveParameter -Parameters ForwardTo, RedirectTo, ForwardAsAttachmentTo

Set-ManagementRoleEntry MyBaseOptions-NoForward\Set-InboxRule -RemoveParameter -Parameters ForwardTo, RedirectTo, ForwardAsAttachmentTo

New-RoleAssignmentPolicy -Name PolicyWithNoEmailForward -Roles MyContactInformation, MyRetentionPolicies, MyMailSubscriptions, MyTextMessaging, MyVoiceMail, MyDistributionGroupMembership, MyDistributionGroups, MyProfileInformation, MyBaseOptions-NoForward -Description “User role assignment policy that restricts the assignees from being able to autoforward email outside the organization”

To set it:

get-mailboxplan | Set-mailboxplan -RoleAssignmentPolicy PolicyWithNoEmailForward

get-mailbox | Set-Mailbox -RoleAssignmentPolicy PolicyWithNoEmailForward