Lesser crimes are merely punished by stripping the angel of their halo and replacing it with a featureless black one that has no enchanted powers. Serious violations of the law, neglect of the angel’s duties or other kinds of unaccepted behavior can result in the angel being thrown out of Heaven. In this case the angel will be granted a black halo that glows red and also black wings. Unlike other halos that serve as passports to get into Heaven, a black halo will keep you out of Heaven.
Other angels are expected to treat a fallen angel as an outcast and are not allowed to interact with them in any way. Demons and other hell spawn on the other hand see the black halo and wings as a green light for attacking the angel, because they know they will receive no help from Heaven.
A fallen angel can redeem themselves by slaying demons or otherwise help Heaven's interests. It's a dangerous task to fight demons on your own, and it's usually the slaying of hundreds or thousands that is required for the angel to be welcomed back into Heaven. This custom has granted the fallen angels the nick name ‘beast busters’, especially among demons.
Officially it's possible for all fallen angels (with the exception of those that fell alongside Lucifer and helped create Hell) to redeem themselves and get a fresh start as long as they do not give up their halo, but it is the opinion of the High Council that some angels are not ever worthy of re-entering Heaven.
If an angel gives up their halo and leaves Heaven for good they're no longer considered a fallen angel, but a devil.