\documentclass[a4paper]{article}
\usepackage{MnSymbol}
\usepackage{stmaryrd}
+\usepackage{slashed}
\begin{document}
\def\iwjtest{{/\mkern-9mu\Sqsupset}}
-\newcommand{\iwjtestx}{\ensuremath{\mathaccent\Sqsubset\sslash}}
+\newcommand{\iwjtestx}{\mathrel{\ooalign{$\Sqsubset$ $\sslash$}}}
+\newcommand{\iwjtesty}{{%
+ \declareslashed{}{\sslash}{-0.04}{0}{\Sqsubset}\slashed{\Sqsubset}}}
sponge
$ A \sqsubseteq B $
$ A \not \sqsubseteq B $
$ \sslash $
$ \iwjtest $
$ \iwjtestx $
+$ \iwjtesty $
+$ A \iwjtesty B $
\end{document}