Locking is done per subscription, not on the menu page:

1. Go to Subscriptions, open the customer's subscription.
2. In the General Settings panel find Lock Menu: and click the value.
3. Choose Yes in the "Lock Menu (no meal selection)" selector and confirm. The change saves immediately.

From then on the meals for that plan are curated by you (via the subscription's re-order settings / auto-selection) and the customer can no longer pick or swap meals. Set it back to No to reopen selection. When the Reorder panel is available, its How to lock button opens a short instructional video.