To remove newlines, use tr:
tr -d '\n'
If you want to replace each newline with a single space:
tr '\n' ' '
The error ba: Event not found is coming from csh, and is due to csh trying to match !ba in your history list. You can escape the ! and write the command:
sed ':a;N;$\!ba;s/\n/ /g' # Suitable for csh only!!
but sed is the wrong tool for this, and you would be better off using a shell that handles quoted strings more reasonably. That is, stop using csh and start using bash.