i can't block win98 computers

[vladone]

Hi!
I try to block some computers to acces my gateway based on MAC
address.
I use this ipfw rule:
  ipfw add 100 deny mac any xx:yy:aa:bb:cc:dd in via $private_interface
With this i can block XP computers but not work with Win98. I dont
understand what is happened!
I try against different computer with win98 OS and i can't block it.
Only messenger is blocked but navigation work well. Computers with
WinXP OS is blocked succesfull.
I believe as is an problem with TCP packets that comming from Win98
computers but i dont know how i can resolv this.