At this time there are two options.
1. You can open and view a recipe. In the upper left there is a Delete link next to the Download link.
2. Or, you can view your cookbooks by selecting My Cookbooks from the black drop-down menu in the upper left of the mastercook.com web page. Delete the cookbook, and it will delete all of the recipes too. Then you can create a new cookbook.
You may wish to create a cookbook just for downloading and delete it after downloading those recipes to MasterCook.