The Fastx.jl bug has been fixed in later versions. However, when I installed Fastx.jl, I only got 1.0.0.
I would try best to make Bio.jl invisible. Extreme actions include archiving github and taking down the documentation website (put a PDF there for remaining users instead). Less extreme actions could be to put a deprecation banner on all Bio.jl related pages and ask google not to index Bio.jl pages. At the very least, write something like “this repo is deprecated” in the github description of Bio.jl in addition to README, and mention Bio.jl is deprecated at Biojulia.net.
As an outsider, I am not sure why you maintain a separate registry. This reduces the visibility of Biojulia packages and makes it more difficult to install them. It may be better to move mature packages to the main registry.