Yes, you are able to accept or decline an ecommerce retailer. Most suppliers see tremendous value in working with as many cool, unique ecommerce retailers as possible, as this dramatically increases market exposure globally.
However, should you decide to not work with a specific retailer, you may reject their dropshipping relationship. You can do this by hovering over their slate, clicking "View Application" and "End Dropshipping Relationship". Doing that will pull your products from their store and they will no longer be able to add new products from your collection unless you reauthorize them.