To charge bad payers interest on their account:
Go to people
Open the relevant person record
Click on more details
Set a percentage in the default interest area
Click save
Go to the People tab
Select Owners in the View dropdown
Click on the owner you wish to charge interest
Go to the Person Details tab
Scroll down to the Financial Information section
Enter the % interest you wish to charge the owner
Press Save
Within the Month End process you will be asked whether you wish to charge interest to selected owners within Step 6. Untick those whom you don't wish to charge interest on overdue balances and then click Process