Currently, there is no way to stop a Workflow for a Contact if the Contact is in the midst of a Workflow.
In conjunction with a trigger to allow users to start a Workflow manually (Shortcut) from the message module, a step to allow users to ejects contact from a specified workflow will be very helpful. For e.g. build a workflow with Shortcut trigger and a step to eject contact from the onboarding workflow.
The step will also be useful if we want to make sure to stop other workflows before continuing with the other steps added in the current workflow.