best way: don’t process these lines at all. that simply means deleting the appropriate code from the style.
example from the lines file:
#this would process national borders, but no other borders
boundary=national | (boundary=administrative & admin_level=2) [0x1e resolution 16])
leave out/delete all other boundary=administrative lines.
for more complex problems, have a look at the delete command, here: http://wiki.openstreetmap.org/wiki/Mkgmap/help/style_rules#delete
