I think what you're trying to do is add extra code to the conditional statements:
if ( !_stricmp(_argv[1], "icmp") )
{ bShowTCP = FALSE;
bShowUDP = FALSE;
}
else if ( !_stricmp(_argv[1], "tcp") )
{
bShowICMP = FALSE;
bShowUDP = FALSE;
}
else
{
bShowUDP = TRUE; printf( "\nUsage lsniff [ICMP|TCP]\n" );
exit(0);
}