I suggested this on a similar thread a while back and it was not well received; but I don't think I explained it very well. If the idea still offends people then just ignore this post and please don't flame me.
This is the hack we are using at our table, it is not RAW.
You can use evocation for a defencive roll *if* you have not already cast an evocation this turn; but you can't cast a block. A block would have duration and would apply to any subsequent attacks and that's too much benefit for being unprepared. We treat it similar to an attack but instead of weapon:x it is parry:x.
If you are defending against a weapon:3 attack then you need 3 shifts (parry:3) to parry it. Your defence is Discipline + 4df. Those 3 shifts (parry:3) are *not* added to you defence roll and they are *not* used as damage if your defence beats the attack.
If you use this option you still get to act when your turn comes up but you can not cast another evocation.