We had a topic about which proof mark people use a while ago, and I was just looking at the common contradiction symbols used on http://en.wikipedia.org/wiki/Contradiction#Symbolic_representation . After doing a logic course I pretty much settled on "up tack" (http://www.fileformat.info/info/unicode/char/22a5/index.htm), probably mainly because it's easy to write.
I've seen $\perp,\nleftrightarrow$, as well as two which don't show up in the LaTeX script. But I would use $\Rightarrow\Leftarrow$, although I just generally write "this is a contradiction." --- To read the math in my posts, download https://addons.mozilla.org/en-US/firefox/addon/748 and http://www.gold-saucer.org/mathml/greasemonkey/.