Assuming that these printers have Lexmark ethernet cards (or have it on the
system board), you can set it up so that it will only make a TCP/IP connection
with those addresses you specify.
Start your browser and then enter the IP address of the printer. This should bring
up the web page built into the ethernet adapter. Go to configuration and there
should be a TCP/IP tab. Click on this and it should allow you to enter IP
addresses that you want to allow to connect to the printer.