You can exclude a specific user from a DD group with the Set-DynamicDistributionGroup
commandlet. Specifically with the -RecipientFilter
option.
Example: Set-DynamicDistributionGroup -Identity MadeUpName -RecipientFilter {((RecipientType -eq 'UserMailbox') -and -not(Name -like 'Journal User'))}
Note that this example is just with recipient type = userMailbox. You can get the current recipientFilter of your DD group and modify it however from powershell.
(get-DynamicDistributionGroup -Identity MadeUpName).RecipientFilter
I have tested successfully!