Watch episode 67 again.
They wouldn't need to seal him up if Beerus could kill him.
Beerus and Whis refused to go to the future, which is why goku and co settled on the mafuba. Whis changed his mind later, but Beerus never did.
All that happens in that episode is whis says is that he has something way better than the mafuba for sealing Zamasu away with no detail given at all. Beerus never interacts with Zamasu in any way other than erasing present Zamasu, and beerus still doesn't time travel.
I don't think you can conclude that hakai doesn't work on immortals from that. (Because it's obviously still possible to kill immortals, another word without its proper meaning in Dragonball, yay)
Edit: the words whis uses in the daisuki sub are along the lines of
Whis:"I will go to the future and ask Beerus to take care of Zamasu"
Only goku doubts if they (beerus) can even kill an immortal Zamasu, and it's then followed up with vegeta saying "even if you can..." Before whis mentions the vague sealing capability he says is way better than mafuba. For all we know, he could have meant hakai.