\documentclass[a4paper,12pt]{report}
\begin{document}

\newlength{\laenge}
\newcommand{\strich}[2][.5ex]
  {\settowidth{\laenge}{#2}%
   \makebox[0pt][l]{#2}%
   \rule[#1]{\laenge}{0.15mm}}

... was zur \strich{Durchstreichung} ...
... \strich[-1pt]{Unterstreichen} ...

\end{document}