You can delete a full bill / sub bill sections with all items from the Bill Summary window or you can delete them individually, there is no bulk delete function.
There is a work around if you are working with multiple items. (For the below Example I will use Bill 1 and Bill 99 as an example)
Bill 1 has 100 items and you only want to keep 10 of them
Create a New Bill heading (use a bill you have not used before) Bill 99
Go back to Bill 1 and highlight all the items you want to keep, Range, Flags, select an unused Flag
Go to Copy Items and copy all flagged items from Bill 1 to Bill 99
Once completed Delete Bill 1 and rename Bill 99 back to Bill 1 leaving you with the 10 remaining Items only.