When you try to delete a bill which was already paid, the system will show you an error message with the information, that it is not possible due to the transactions assigned to the bill.
In order to delete a paid bill, you need to delete the transaction first. See the instruction below.
Double click on the selected bill in order to open it and show the details.
Open Bill Payments tab in order to see the list of transactions assigned to the bill.
Select the transaction and click on Edit transaction button.
In Edit Transaction window, change the amount to 0 in order to remove the transaction and save the changes.
The bill will appear as UNPAID.
After that, close the bill and delete it from the list using Delete Bill button.