IPFW firewall rule in mpd pppoe server to single pc behind router

[Gloomy Group]

Hello all,

  
 I have freebsd mpd pppoe server. Users connect to internet by giving username and password. My problem is some users put router and share internet connection with other pc. Is it possbile to disable internet sharing in server by rate limiting with ipfw firewall scripts. So that if users keep router or does nat in their pc to share internet then only single pc can access to internet.
Is is possible?

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