Assuming you want to divert these emails and never send them to an agent, you can access the email's subject in the Inbound Email flow with the built in variable Email.Message.subject
So you can either hard code the subject into a decision block, or create a Data Table to contain the subjects you want to divert and then do a lookup, if the subject is not found you can route the Interaction normally. If you want to get really fancy, you can put the "canned" reply for each subject into the Data Table in a second column.
HTH
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Paul Simpson
Views expressed are my own and do not necessarily reflect those of my employer.
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