These sources can be disabled completely or for all managed users in Settings → Online Media Sources.
Feature Request: Please make these sources selectable for managed users either on the Settings → Online Media Sources page (add users for selection) or as additional libraries on the managed user’s restrictions page.
You’re in the right spot… though not all topics get the immediate attention of the entire community (or meet their preferences).
btw… you might want to consider voting for your own suggestion. Suggestions with 0 votes imply that not even the OP is actually interested in them
I took the liberty to slightly rephrase your suggestion’s title to make it easier for others to find.