mod_bandwidth can limit the flow of data on a per user or per site basis. However I think you're asking: if someone hits "x" bandwidth, can I limit them to "y" xfer speed after that. If that's the case, I'm sure there is but off the top of my head I can't think of anything.
This Apache module is intended to reduce the load on your server & bandwidth generated by popular virtual hosts, directories, locations, or users according to supported polices (see below) that decide when to delay or refuse requests. Also mod_throttle can track and throttle incoming connections by IP address or by authenticated remote user.
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