Joomla just recently (3.x only I believe) started adding the message type as part of the message. It used to just be the CSS class for the message. It's surrounded by JText call so you should be able to translate it within your Joomla language files. Example as follows.
SUCCESS="OK"
Alternatively it has a CSS class so you could add the following to your Joomla template CSS file to hide it using CSS.
Code:
#system-message .alert-heading {
display: none;
}
krileon
