Hahaha, we need one of them minecart accelerator folk to get on the case!
I know the first collision knocks out the rider, but noone said the rotating noble needs to survive
The noble really should. You get another noble who has more mandates if you kill them. The objective is to render the noble incapable of issuing mandates altogether, which means putting the noble in some kind of stasis.
Putting them in a tree where they can't get down, for example, would prevent mandates, although that might also include starvation.
If you can keep them perpetually in a coma, and being fed and watered by some sort of nurse dwarf, you keep them from ever issuing a mandate and still alive so that nobody sends a replacement. Syndromes are a good place to start with this, although it needs to be very long-term or else set up with easy repeatability to really work properly. (I guess dumping syndrome extract on a lever nobody else is allowed to path to so it can't be cleaned is an idea, although the problem is, the noble can issue mandates in the brief time between waking up and pulling the lever again, and if the timer is ready for a new mandate, they may well issue a mandate the instant they wake up.)
Much simpler is using silk farms where the king is the target of a GCS, and leaving him nigh-permanently webbed. Minecart loops involving lots of jumps could also work. These have the problem of requiring you let the noble out to feed them once in a while, however.
That said, just finding a mayor who likes granite on a map with tons of granite so you don't have to worry about it is much easier than most of these options...