Here is how to hide it.
Go to "Look & Feel" -> "Custom JS", enable it and add the following code:
Code:
TopMessages.prototype.messageHandler = function (text, type) {
if (type != MESSAGE_INFO) {
this.addRecord(text, type);
}
};
Looks so much better with out it! Now you just see the item fly into the cart with out the big green box telling you it just went it the cart lol.
Just so you know it hides it for everything though including errors.