For a flow not synchronised with a SIS-system:
You can only remove a participant as long as the participation date has not been reached. You remove a participant from the flow by navigating to Participants > Participant details and then pressing the big green minus. You will be asked to confirm this:
If the participation date has been reached, it will only be possible to deactivate the participant on the flow.
For a flow synchronised with a SIS-system:
When a flow is synchronised with a SIS-system, it is only possible to deactivate them. A deactivation means that they are no longer notified about the flow and cannot access the flow. Anything they have submitted is still saved. You will be able to deactivate a participant at Participants > Basic information by unchecking the small checkbox at the participant.