There is a cure for immortality per the American movie classic that won all the awards for the best movie, ever. Highlander of course, and this gives me a moment in time to scratch something off my bucket list. *use Scary Movie 3 quote in a discussion* check.
Granted we have no shovel, and we're not talking about aliens but...I found their weakness. They're powerless without their heads!
fighting an immortal? Lop that head off.
Sadly, there are always different grades of immortality.
For example... I forgot which of the Norse gods it was, but nothing could kill him except holly. And Loki slays him with a sprig of holly.
So you can't just say 'Immortals will die with no head!'
It could be 'Immortals will be inconvenienced with no head!'
Or 'Immortals... we can't even cut off their head.'