This post will show you how to turn off spell check in safari. You can also turn off auto-correcting using this method.
click in a text box or text area (the addressbar will do)
goto edit > spelling and grammar
untick ‘check spelling while typing’ to get rid of the red underlines when you mis-spell words
untick ‘correct spelling automatically’ to stop safari from replacing words it thinks are incorrect.
I generally keep the check spelling while typing option on so that I know when I make a mistake, but it doesnt correct things that its not meant to!