Can the clients still edit their contacts after they have submitted the questionnaire?

The client will still have the option to edit the questionnaire until you lock the matter for further changes.

However, your client will no longer have the option to delete contact after submitting the questionnaire. Firm staff users will still be able to delete the contact for the client. 

To delete a contact you need to open the 'Client Intake Form':

Go to the 'Contact Details' section (or however you may have renamed that):Then, click on the 'Delete' button on the contact you need to remove.