This is not possible nativley with the MWG.
Bandwidth throttling on the MWG will only attempt to limit the bandwidth usage on a per connection basis, this means that it does not take into account all the connections for a specific user as a whole.
So a user could have 1 connection, where you would throttle it down to X kbps.
But the user could open 20 more connections and each of those connections (individually) could be limited to X kbps (20*X=20X kbps), not all 20 connections limited down to X kbps.
Let me know if that makes sense.
Here is other discussions on the topic, as well as the whitepaper: