Which browser are you using? If it's Firefox, click "Tools" at the top, choose "Options" in the dropdown menu, then "Advanced" in the menu along the left, and from there you can untoggle "Check my spelling as I type" (also "Search for text when I start typing").A bit off topic but does apply to posting messages here too. How do I turn off that evil, sneaky, sometimes helpful auto-correct?
For Chrome, you want to click on the three little dots at the upper right, choose "Settings" in the dropdown menu, then "Show advanced settings" at the very bottom. Under Privacy, you can untoggle "Use a web service to help reserve spelling errors".