You can prefix most mathematical symbols with \not in order to negate them. The following looks good according to me:
\documentclass{minimal}
\begin{document}
Did you know that $\mathcal{A}\sqsubseteq\mathcal{B}$? However, $\mathcal{A}\not\sqsubseteq\mathcal{C}$. Also, $\mathcal{C}\not\subseteq\mathcal{D}$.
\end{document}
Alternatively, as suggested by @Alan, there are some variants to this (as well as complete symbols) as part of txfonts/pxfonts, mathabx and mnsymbol packages. See the page 37 of Scott Pakin's Comprehensive LaTeX Symbol List for more details.
Best Answer
It's the standard
\mathcal{K}
.I guess that your document uses
mathptmx
. Do like this:If you're using
newtx
, the code should be