You can manually change the opt-out status of a contact in the Contact Manager.

• Goto Contacts
• Search for the mobile number +44756.... or the email address or the name of the contact
• You'll spot an opt-out column on the right-hand side of the manager. 
  • The status will be 'False' if they haven't opted out already. 
• Double click to activate.
• Tick the box (making them opt-out)
• Click outside the opt-out column and it will turn the opt-out status to True for this contact

Once 'True' they will not receive a message from the platform.