We rarely recommend deleting contacts! If a user has requested to be opt’d out it is best practice to lock them so they can’t be texted but you have the message and contact history still visible for recording purposes.
To delete or opt out a user you need to open the conversation and hit the Contact Info icon where you will find the trashcan to delete or the lock to opt out.
Reminder: if you delete a contact, you will delete their entire chat history. Even if you add the contact again, their conversation will not reload.