Removing the line and address from scrlttr

To remove the line and the text, add backaddress=off to the class options. To remove just the line, add backaddress=plain to the class options.

Set the KOMA variable backaddress to be empty:

\setkomavar{backaddress}{}

or see Michael's suggestion.

To remove the reserved space, set the pseudolength backaddrheight to 0:

\makeatletter
\@setplength{backaddrheight}{0pt}
\makeatother

Related