How exactly will they reverse the spell?

Last time we saw it, it was reversed by casting Dispel magic. I was a bit surprised, because Dispel magic counters ongoing effects. Baleful polymorph is described as "permanent", yet if the creature fails the Will save after 24 hrs, I thought it was a kinda really permanent change. The spell description doesn't say exactly how it may be removed, if the permanant change following a failed Will save has occurred. I kinda thought it might be impossible to remove, then. It's not a curse, or is it?

Now of course the author may reverse the spell any way he likes. But it would be interesting to speculate how.