documentclass{scrlttr2}
setkomavar{fromname}{Seamus}
setkomavar{fromaddress}{Earth\
  Solar System\
  Milky Way\
  The Universe}
begin{document}
begin{letter}{TeX\
    Stackexhange\
    dot com\
    The internet}
  opening{Dear TeXnicians}

  I am having trouble with the above ruled line

  closing{Yours hopefully}
end{letter}
end{document}

setkomavar{backaddress}{}

makeatletter
@setplength{backaddrheight}{0pt}
makeatother

KOMAoptions{backaddress=false}

documentclass{scrlttr2}

KOMAoptions{backaddress=false}

setkomavar{fromname}{Seamus}
setkomavar{fromaddress}{Earth\
  Solar System\
  Milky Way\
  The Universe}

begin{document}
begin{letter}{TeX\
    Stackexhange\
    dot com\
    The internet}
  opening{Dear TeXnicians}

  I am having trouble with the above ruled line

  closing{Yours hopefully}
end{letter}
end{document}