To delete items in your catalog go to the Setup Items section and press the "Import/Export" tab.
Select the items you wish to delete and click the red "X".
You can use the Search tool to help you isolate the list to items that you want to delete.
Deleted items can be recovered from the Recycle Bin.
NOTE: When uploading Excel files of items, if you upload a file that does not contain certain items already in your catalog THEY WILL NOT BE DELETED from your catalog.