I don't think there is really a direct way to do this. However, If you are sending reminders from this process, you can include the "Update:" in the subject so when the users reply back you ensure the process is just being updated an no new records are being created.
The note replies with the key word "Update:" work properly. When anything else without the keyword "Update:" come in it creates a new record even without the Title and Description mapped. This feels like a bug to me.
What happens when you try to set the default value of the title to be "Update:" ?
I've seen this logic too and suspect this is a design "feature" rather than a bug.
It would be nice if ServiceDesk sent back a nice message rather than create when not mapped for sure.
Perhaps as a workaround you could setup an exchange rule though (with reply message) to automatically reply to the user encouraging then to use self service if the subject line doesn't include the keyword "Update:" and also move to another folder so that it doesn't process and create the lifecycle as the exchange rule should fire before it hits the mailbox?
Stupid question, is Title Mandatory? If it is mandatory then it should complain about not being able to leave the Title field null? At least I'm sure that's how it used to work.