it's not an option to control borders.
it's an option to SUPPRESS THE SECTION. in the section, you have a line. if you suppress the entire section, the line doesn't show.
also, you can't use an IF statement in the suppression formula. think of the suppression formula this way:
i want to suppress records where....
not onlastrecord or isnull({field})
in other words, it's the logical opposite of your IF statement... because you want to suppress that section for every occurance but the last record or null records.
also, regarding your border formula... have you verified that your last two records are in fact NULL and not a blank space or "" ? in order for isnull() to work, the field must be NULL and not just empty or blank.
and... you could always get rid of your null records at the end by creating a Record Selection formula that excludes null records...
