If desired, you can delete contacts from Act-On Contacts. Deleting is permanent unless the contact is in your CRM system, in which case they will get synced back to Act-On Contacts on the next sync. 

Instructions

Delete One Contact from a Segment

1. Navigate to Contacts > All Contacts > Contact Listing
2. Hover over the contact row
3. Click More Options  
4. Click Delete Contact

Delete Multiple Contacts from a Segment

1. Navigate to Contacts > All Contacts > Contact Listing
2. Use the checkboxes to select multiple contacts
3. Go to the blue bar located at the top and click Delete Contact 

Delete All Contacts from a Segment

1. Navigate to Contacts > All Contacts
2. Hover over the segment
3. Click the More Options  
4. Click Delete All Contacts