I had some type of valonia or relative to in my first tank. (looked like sausages but was quite hard to get off the rocks and any bit left turned into more.) Horrible invasive stuff, along with caulerpa, bryopsis.
I have to say +1 on the blowtorch! I got sooo sick of this infested rock I torched every outside area and fried every bit of it off. Then I left the rock in a circulating tub in the dark for months and did several WC's (lots of die off). Later there was still some life to the rocks, just nothing that had been on the outside of it. (as in I found some pods in there, dusters etc.) The coralline died of course but I think the "inner" rock lived as it did not get hot, just the surface did. I understand the whole problem of "1 spore" getting in, however if your problem is this bad and you're already considering it.... You've been in sw long enough to know the bio-ramifications of doing extreme things with LR (cooking, etc) as well as adding critters, so I won't bother even talking about that. Granted this doesn't guarantee the rock will be free of it forever. All I can say is that on that particular bunch of rocks, it killed it, it has not come back and I got to burn something.
(Oh and +10000 on personal satisfaction of burning. Die algae die!!!)