For a flow not synchronized 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 at the bottom at "Participants" > "Participant details" and then pressing the big green minus. You will be asked to confirm this:
If the participation date is reached it is only possible to deactivate the participant on the flow.
For a flow synchronized with a SIS-system:
When a flow is synchronized 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" and then by unchecking the small checkbox at the participant.