If a contact has removed consent to receive communications in a given manner for a given purpose you will need to update the purpose for the contact to show that consent has been removed.
đNote: You are not able to delete the purpose as this is not inline with GDPR.
To update the consents on a Purpose:
Find the contact record and click the Contact Preferences tab.
Locate the Purposes sublist and click + Add.
This will allow you to add a new set of consents to the purposes.
Select the new status against each communication method.
Select the Purposes to record the new consents against.
Click Save.
This will update the consents and purposes to show the new status.