Unfortunately when you choose disable in Retail Express or turn off Export to Web it means the product disappears from the integrations view and can no longer be seen by the integration at all. Worse it will break any variable product relaying on all these sizes being together,
Before disabling or marking not export it is very important you make sure the product has a zero stock quantity and is made not visible on the website.
If you do not follow this process :-
1) The product can still sell from the web store but the order can not be created in Retail Express
2) The price will be stuck at what it was last in Retail Express forever.
3) No Smarttags or other logic can work.
4) The integration will add this to the "Missing in REX" component of your comparison report.