I don't know if it's possible for you but a freshwater (same temperature as tank temperature) dip for 3 minutes will kill the nudibranches (and any other inverts) but will not harm the zoas. But if the zoas are on large pieces of rock then you will not be able to do that as you'd destroy too much of the beneficial bacteria and creatures in it. The freshwater dip will only kill hatched nudibranches. Any eggs may still survive.
Good luck.
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