Hi Lynda,
It sounds like you have a suspicious client. I recommend asking your payment processor to investigate possible fraud to try to avoid a potentially costly chargeback.
I recommend changing your administrator password to something longer (ie. 10 characters) and unguessable (containing a number or a capital or two).
To ban the client from ListMailPRO by IP you'll have to manually modify the admin.php file. Add this line on a blank line after the opening <?php tag.
if($_SERVER['REMOTE_ADDR']=='99.245.156.44') exit('Please stop hacking.');
To ban them just from login, do the same but in the login.php file.
A ban feature is coming in a near future update
Regards