Hello @fairsailing,
Welcome to the LastPass Community!
If you with to delete certain sites and passwords. It depends on what's your goal but if you no longer want to have that account in that specific website you will need to delete the account in the specific website and then delete it from LastPass. If you delete it only from LastPass it will stop being save there but the account will continue existing. You can find more here: https://support.lastpass.com/s/document-item?language=en_US&bundleId=lastpass&topicId=LastPass/t_lp_edit_or_delete_an_item.html&_LANG=enus
Kind Regards,
Alejandra
LastPass Support