Holding the shift key down at login should prevent apps from relaunching for that login.
If you want to permanently prevent apps from reopening, you can quit all apps then lock the
com.apple.loginwindow.*.plist file found in
This plist keeps a list of open apps, and any apps that are in that list at logout will be launched again after login (assuming the checkbox is ticked to do so, as it always is by default). If you quit all apps, then lock that plist, it's list of apps will always be empty, so even when that checkbox is ticked, there will be no apps in the list to relaunch.
Unlocking the plist will return it to normal behavior.