I think it's in the core rules under Wizards
"If a spell is cast, the opposing player can
choose any one of their wizards that is
within 18" of the caster, and that can see
them, and attempt to unbind the spell
before its effects are applied. To unbind a
spell, roll two dice. If the roll beats the roll
used to cast the spell, then the spell’s effects
are negated. Only one attempt can be made
to unbind a spell."
So, you can only make one unbind attempt per spell cast, but other abilities such as ignoring a spell cast on this unit on a 5+ etc like on The Hallowed Knights battalion warscroll, and some allegiances have an artefact that can possibly ignore a spell's effects.