In text environment you can use <strike> </strike>, for example: strike.
In math environment, you can cross out expressions using cancel bcancel xcancel or cancelto as long as you require them first.
$$\require{cancel} \cancel{2-2}$$
$$\require{bcancel} \bcancel{2-2}$$
$$\require{xcancel} \xcancel{2-2}$$
$$\require{cancelto} \cancelto{0}{2-2}$$
As far as I know there is no perfect execution of horizontal strike-through in math environment. You can try to do it using css, with \style{text-decoration:line-through}{...}. This works fine sometimes
$$\style{text-decoration:line-through}{abcd\sum\prod\bigotimes}$$
but it messes up with subscripts and superscripts and fractions
$$\style{text-decoration:line-through}{\sum_{i=0}^n\left(x_iy^i\frac{z}{u}\right)}$$
I hope someone knows of a more reliable way to do horizontal strike-through in math environment, but I think this may be it.