You can restrict access to an individual sub-account that contains your company's domains. Doing this will restrict access to all settings for these domains, as well as lock down access to the shared quarantine. Only those account contacts that are marked as "owners" of your reseller account will have access.

To do this, go to Reseller > My customers, hover over the row containing your domain (you can use the search in the top right to find it), click the restrict access button that appears, and confirm your decision by clicking the Restrict access to... button on the page that follows.