No more going into the file and deleting the entry manually.
To remove line 123 from ~/.ssh/known_hosts:
Usage
rmknownhost 123
Code
Put this it into a file called rmknownhost inside a folder that is in your PATH and chmod +x the file.
#! /usr/bin/env ruby
line = ARGV[0] || raise("gimme line to remove")
hosts = File.expand_path("~/.ssh/known_hosts")
content = File.readlines(hosts)
removed = content.delete_at line.to_i - 1
puts "Removed:\n#{removed}"
File.open(hosts, 'w'){|f| f.write content * ""}
Michael Grosser, the Blog 
You can just write it in shell
sed -e “123d” -i ~/.ssh/known_hosts
That’s how I do it:
sed -i ‘123 d’ ~/.ssh/known_hosts
wow thats short 😀
the “Removed: xxx” message is missing but thats not that important.
Its a bit cryptic, wrapping it into a function would be good…
If you know the hostname, you can also use:
ssh-keygen -R