Since deleting an Alias after it's been used is impossible I think having the option so set one as inactive thereby removing it as an option when posting would be a useful tool.
At the very least it would keep the "Post As" drop down uncluttered for those who have 15-20 or more aliases.
I agree! It's already been on the wishlist for forums for a while. So once the tech team gets to working on the forums again, we'll see how it shakes out :)