No-one has taken away those rights. Private institutions have banned him from using their platforms or services because he undeniably violated their terms of service regarding spreading hateful and fraudulent claims, as is their right.
If those on the alt-right want to remain on these platforms and services they’ll just have to be more circumspect and subtle about spreading their lies and bigotry (I’d suggest they refrain from doing so entirely, but this is the alt-right we’re talking about).
