It’s a Minor Miracle (Ob 5) to interrupt a spell, but a Hindrance (Ob 4) will paralyze the spell caster, which naturally blow any spell-casting in progress.
What to do?
a) Good point, using Hindrance is clever.
b) Nah, Hindrance will stop you swinging a sword, but it’s not a May Not for spell casters. Make the priest hit Ob 5 to interrupt a spell.
This came up in the game last night - Dallin used Hindrance to interrupt Ghellut the Bloody casting Somatic Traitor. Seemed appropriate given that Ghellut’s idiom is waving his arms around. As it happens he made enough successes to beat either obstacle, so it doesn’t matter, but in discussing the game afterwards, Zelbinian pointed out the text suggests Minor Miracle for interrupting spells.