In order to delete an object, all associated objects must first be deleted.  For example, if you are deleting a user (the first admin user cannot be deleted), ensure that the user being deleted is not a primary user in any of the cases (click on the click under "Cases (Projects), then click on "Case Details/Edit."  Change primary staff to another staff.  Check all cases.  Then ensure that there are no time/expense entries by this being deleted user/staff. Either delete those entries or edit them to change the staff name.  The system will now let you delete the staff/user.

For deleting a note (time/expense entry),  the note should not have been included in any invoice.  If so, you may delete the invoice, if so desired.  If a payment has been recorded against an invoice, you will need to delete the payment before the system will let the invoice to be deleted.