Script needed - Remove all server-side forwarding from mailboxes where the AD account is disabled

It's a bit more challenging to do for disabled accounts but you stated the accounts are disabled and the mailbox hidden from the address lists.  In that case, lets target those mailboxes that are hidden from the address lists.

For all mailboxes with the HiddenFromAddressListsEnabled parameter set to true, we will disable all forwarding like this...

Get-Mailbox -ResultSize Unlimited | Where { $_.HiddenFromAddressListsEnabled -eq $true } | Set-Mailbox -DeliverToMailboxAndForward $false -ForwardingSmtpAddress $null -ForwardingAddress $null

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If you want to do it for a list of users in a CSV file (with DisabledUser as a header), we will do it like this...

Import-CSV "DisabledUsers.csv" | foreach-object { Get-Mailbox -Identity $_.DisabledUser | Set-Mailbox -DeliverToMailboxAndForward $false -ForwardingSmtpAddress $null -ForwardingAddress $null }

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Hope this helps.