You can block a product(s) through the Products section.

Go to 'Products > All Products’ then select the product/products that you wish to block by checking the check box on the left. Click the drop down on the ‘Actions’ button at the top and choose ‘Block all selected’.

When you have finished blocking and want to push the changes to the marketplace, go to the dashboard and click the 'Upload To ....  ' button at the top.

If you would like to unblock an product the procedure is the same, just choose the "Unblock all selected" from the dropdown