If you want to delete your default Master List, but save items that are currently in your custom Lists, please do the following:
Make an iTunes backup
Go to any list, tap "+" to Add New item
Click "Done" to hide the keyboard
Click "Edit", choose "Select All"
Click "Delete", confirm "Proceed"
This will delete All items in you Master List, but will not affect items that are currently in your custom Lists
To delete item from your custom list:
Delete it from your custom list swiping it off in "Shop" mode or setting item amount lower than zero in "Prepare" mode
Then repeat Delete Master List procedure
Please NOTE: By Deleting Master List you will delete ALL ITEMS THAT ARE currently NOT IN YOUR custom LISTS
To restore default Master List:
Sign up for a New Group (unless you are already in a group)
Reinstall the app
Join your group
Choose "Merge the app and group" option