How to block further changes by the client on a questionnaire?

Open the matter, and on the right-hand side of the Matter Details screen set Client users can make changes to No:

When the client logs into the app they now see that the questionnaire is locked:

Frequently Asked Questions

Can my client still log in while the questionnaire is locked?

Yes. They can still log in to the portal, but the questionnaire shows as locked and they cannot make changes.

Can I unlock it again later?

Yes. Set Client users can make changes back to Yes on the Matter Details screen and the client can edit again.

Is this the same as setting the questionnaire back to unsubmitted?

No, it is the opposite. This setting locks the questionnaire against changes. Setting it back to unsubmitted reopens it for edits and a new submission; see the article How do I set a questionnaire to unsubmitted.