Well, the easiest way would be to select all people in the SL form the people tab > Person Actions > Special > Remove from Flow > In the dialog with Remove from Flow step - select the Campaign from which you want to remove people (i.e., your nurture send campaign) > Hit Run. This'll remove people from the campaign flow immediately.


Alternatively, you can also create a batch campaign referencing the SL created in to the campaign's smart list tab (you would want to use Member of Smart List in <select your smart list> filter), in flow drag the Remove from Flow step in to the canvas, and select the Campaign from which you want to remove people (i.e., your nurture send campaign). Run the batch campaign using from the schedule tab of the campaign (this'll remove the people from campaign flow immediately). Also - instead of using the nested smart list (i.e., referencing the smart list you created in to the campaign's smart list) - you can also build out the SL using the same filter criteria in the campaign's SL directly. 🙂
If you're gonna be doing this exercise of removing people more than once you can go with the smart-campaign approach just so that you have log of runs at one place.