It looks like that Journey branch (in a Yes/No split for example) that has no activity and leads nowhere... IS an Exit of the journey.
So, a solution would be to check for an event (purchase for example). If the event:paying_user is present, do nothing, if not then continue the journey and send the next marketing emails.
A little burdensome, as you have to check before every next Communication Action, but this is what I found so far.
Correction: Any Journey branch that leads to no further actions is a Journey Exit.
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