If you are not looking to use the captcha in your Joomla contact form or login, registration form, etc, then here is how to do it.
Goto Plugin Manager . Find Captcha Plugins and switch all of them OFF.
Goto User Manager > Options popup. Set the default Captcha to NONE SELECTED.
Goto Global Configuration. Set the default Captcha to NONE SELECTED.
You are done. Clear your Joomla cache and browser cache and test the forms again. You forms should not show the captcha thing again.