You should be able to head over to Settings>Bindings>IP Addresses, select your IP addresses one at a time and uncheck the POP ports listed here to remove POP access from the interfaces SmarterMail is bound to. If this doesn't work I recommend submitting a support ticket so we can take a look, or blocking the port at the firewall level as the poster above pointed out. Let us know how it goes when you get a chance to test.
Kyle Kerst
IT Coordinator
SmarterTools Inc.
www.smartertools.com