Hi Mariette,
It is difficult to run workflow to deactivate the event registration, because of workflow can only capture information of contact, while we want to get a specific event record from context.
(When a workflow is executed by customer journey, it can only get submitted form data from contact.)
Instead, you could work with custom channel: create a lookup field to associate a specific event to the custom entity for custom channel,
in flow, retrieve the event registration record whose event field equals the value of lookup field of custom channel activity, then set status field of the registration to Inactive to deactivate it.
Finally, only create the custom channel activity on condition of specific link clicked.(e.g: A link with text "Cancel your registration".)
Regards,
Clofly
