@Pierre1 one other important note expanding on @David's response, while we have traffic limiting in Mbps per client, the situation you're calling out can potentially be where the troubled client is bad enough it isn't reaching it's committed minimum rate, which would then not trigger it to limit airtime on the medium yet.
To solve that, in our TDMA multipoint protocol (which is now being tested on the B5c and soon is migrating to A5), when we reach full airtime slot utilization, we will start to even out timeslot distribution dynamically to the active clients. This prevents a single troubled client from over-occupying the air. So you'll definitely get this a pretty sophisticated auto-throttling, albeit not yet a percentage control, but it would even out timeslots based on the total clients trying to access the medium at any given time.
Hopefully this will address the issue for you in entirety. I think we'll be adding in some obvious health notifications for subscribers with such noticeably low Mbps-to-Airtime ratios to indicate to you the individual client is causing other people problems, and requires resolution.