For some versions of Adobe Reader (maybe all), unchecking "check for updates" (if it's actually present in tools/options) doesn't work because a separate application (Adobe Updater, as mcdhu says) does it as well.
If you really hate Adobe Updater, you can simply scout around the Adobe folder looking for something called "Updater 5" or similar and wipe the directory (caveat, I've never done this but I'm happy to watch you do it).
Otherwise, you have to run the updater manually once to stop it doing its thing. In the "Updater 5" (or whatever) folder, click on AdobeUpdater.exe to start it. There will be a button or underlined text marked "preferences". Click on that, and you should be able to disable automatic checking, and also the installed products it checks for.
Alternatively, in the Adobe product under Help, if there's an option called "check for updates now", that should start the updater running.
Since I don't know the version of Reader you're running, I can't point you to the specific solution. The way to kill the idiot updater depends on the product you have, the version, the time of day, and which way the wind's blowing.
A-"You need to read a PDF, so you're bound to want us to bog your computer down with tons of other junk"-dobe.