All Samsung devices (as far as I know and can tell) have the option of "None". Are you saying it isn't there on your tablet? Or that you just can't find it?
There isn't a key for it, it would be found in the list of lock options. Just tap on the one you already have it set (say it is set to "Swipe" for example), then on the following screen choose and change it to which one you want it to use.