[ic] limiting credit card types accepted?

[interchange-users@interchange.redhat.com (Melissa)]

I have a functioning site, which I thought was configured to accept 
only Visa & Mastercard.   However, orders are successful when 
Discover numbers are entered.

ord/checkout.html correctly displays just Visa & MC images.

Other settings I've checked that seem ok  from the Admin web interface:
    Administration -> Payments
    Tables -> variable -> CREDIT_CARDS_ACCEPTED


I'm using Interchange 4.8.3.  My site is standard configuration, 
built almost entirely through the CommerceLauncher & Admin web 
interface.

Is there a setting to turn on credit card type validation that I've 
missed?  Or does this require additional coding?

Thanks for any suggestions you might have.

Melissa