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 click on "Open Questionnaire".
Go to the 'Contact Details' section (or however you may have renamed that):
From there you can either Edit or Delete any contact in the matter.
After editing the contact details hit Save.