Disable the message shown when a node is created
It's node_form_submit that is creating those messages. You could pretty easily use hook_form_alter on the node form and use your own version of node_form_submit instead. All you would need to do, would be to copy the function and add an user_access('whatever') check before that message is created.

Alternatively, you could in preprocess_page function, check which messages is being served, and remove unwanted ones, but that would be a bit more tricky. Should be possible with some regex. On the other hand, this method would be a bit more upgrade friendly, since you could remain using the node_form_submit function and would get future changes if any.