You can go any father and kill the dns response if your goal is to block line fully just place a ips sensor for " gm.line.naver.jp" and with allow. Than block all other dns request.
config ips custom
edit " naver.jp"
set signature " F-SBID( --name " NaverDnsBlk" ; --rev 1; --protocol udp; --service DNS; --flow from_client; --byte_test 1,<,128,2; --pattern " naver.jp" ; --context host; --no_case)" naver.jp\" ; )"
end
Apply a rule for your dns traffic and a security policy. This with the above HTTP traffic sign should squash all other traffic. Just monitor for a few days/weeks .
It would be nice if fortigate had some response forging or spoof' ing so you could redirect them to page that explain your company policy or AUP.