1. If a wired connection then go via a 10/100 unmanaged switch, that'll reduce its speed by a factor of 10.
2. If WiFi force it onto a 2.5GHz channel. This may be a bit tricky with the THG3000 router, but if splitting the WiFi and giving each a different password you may get somewhere.
None of this is a perfect solution and if there's a "clever" someone in the house they may be able to circumvent these measures.
Short of getting a third party router Cynric's answer is about as good as you are going to get. Being honest, most QOS/bandwidth limiting solutions are either a bit clunky or simply don't live up to the hype! *QOS (Quality Of Service) works far more effectively on the upload side than on the download.
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