You take an action that becomes a Block on their next discipline control roll. You basically substitute a higher difficulty.
So a careful ritual caster might try to gather 1 shift of power each exchange, but if you make a Fantastic Weapons rolls (to throw a rock at the ritual), their next control check is 6 instead of 1.
Nice! This means that a highly disciplined spellcaster might be able to carry on through with the ritual despite the interruption, but at the very least it makes it more difficult to 'concentrate'.
One other thing to consider is that if the ritual does require physical components, and you destroy them, then this might require the caster to make up the complexity 'deficit'
immediately or suffer the consequences of the spell failure. For example, if I used a Summoning Circle In My Basement to give me two shifts toward the complexity requirement, and that circle was damaged and thus made useless for the ritual, then I'd probably need to either invoke an aspect (assuming I had a Fate point and an untapped aspect handy), or suffer a mild (or greater) consequence.