There were many questions in this site about slaying dragons, butchering dragons, but never one about restraining them. Poor Gyvaris managed to avoid all those fates but the last.
Gyvaris is a dragon, currently in the "service" of not St. Martha and not St. George. Having been nearly killed and unintentionally publically humiliated by, then forced to serve them, Gyvaris is unstable.
Stories of dragons razing entire cities by themselves is just unsubstantiated dwarven propaganda (they'd blame everything from missing homework to the Armenian Genocide on dragons if they could get away with it). Regardless Gyvaris' hissy fits could still range anywhere from property damage to burning children.
That is problematic because Gyv is supposed to be a layer of defense for George's village and that limits restrainment methods. It's certain though that you'd want to shut him down IMMEDIATELLY if things are going south.
Dragons are roughly the same size as a horse and as intelligent as humans. Their natural weapons are their jaws, claws and a spade at the end of their tail. They're slim and athletic with enough strength to overpower a human but not bigger animals (i.e: bears). Their breath weapon is basically an explosive charge (think of thermobaric grenades), the shot limit is 5. Under normal condition, the dragon can regain 1 shot per hour.
How could a dragon be restrained in a fast, reliable way to prevent him from damaging others or himself when throwing a tantrum?
The tech-level is late-medieval.
Dragons can fly but we're assuming they choose not to when in a fit.
This question is about restraining dragons, not killing them, if they disobey orders.