Dear Siva,
FBL5N works fine with auth. object F_BKPF_GSB.
Of course a doc. without Business Area will not be restricted, checked against authorization. Maybe you have to make it obligatory during doc. entry.
I suggest you go through and check yr role containing F_BKPF_GSB.
Activate system trace to see the author. object check.
Check also with Tcode S_BCE_6800142 to see a specific user, and for F_BKPF_GSB authorizaiton object (value XXXX activity 03) which role gives access (where XXXX should be the Bus. Area that the user should not have).
Good luck.
BR
Elly