Crossing out arrows

Have you tried the cancel package:

usepackage{cancel}
...
cancel{text}

You should also see the solutions on How do you superimpose two symbols over each other?, and the solution to Diagonal strikeout starting too low and ending too high.

Another option is something like $mathrlap{to}times$. If you add usepackage{xcolor} then $mathrlap{to}{textcolor{red}times}$ might be a better option.

Most mathematical symbols allow negation via the not prefix. For example, consider the look of

f(x)not=frac{-bpmsqrt{b^2-4ac}}{2a}nottomathcal{A}

There is also centernot which looks similar to not= in the above case, yet different (perhaps not-so-good) from notto.

Alternatively, Scott Pakin's Comprehensive Symbol List has a vast amount of mathematical symbols and their associated fonts/package inclusions.

Found via detexify:

usepackage{ amssymb }
nrightarrow