You can remove contacts from a Smart Flow in two ways:
1. From their individual contact profile
You can remove a contact from a Smart Flow from within their contact profile. You will see a tab titled 'Smart Flows' within their profile beside the Timeline tab.
From here, you will see any current Smart Flows that the contact is enrolled in. Click on the 'Remove from Smart Flow' link. You will see a pop-up to confirm the removal of the contact.
2. Audience tab within the Smart Flow
In the Audience tab of a specific Smart Flow, you will see a list of all of the contacts within the Smart Flow. You can check off any number of contacts and you will be able to click on the 'Remove' button under the Actions column. This will remove the contact(s) from the Smart Flow. You will see a pop-up to confirm the remove of the contact(s).