\documentclass[a4paper]{article}
\usepackage{MnSymbol}
\usepackage{stmaryrd}
\begin{document}
\def\iwjtest{{/\mkern-9mu\Sqsupset}}
\newcommand{\iwjtestx}{\ensuremath{\mathaccent\Sqsubset\sslash}}
sponge
$ A \sqsubseteq B $
$ A \not \sqsubseteq B $
$ A \nsqsubseteq B $
$ A \Sqsubset B $
$ A \Sqsupset B $
$ \sslash $
$ \iwjtest $
$ \iwjtestx $
\end{document}
