After publishing, prevent editing

Hello everybody, I need a particular feature: a user of a role can edit node of certain type only if the node isn't published, when the node will be published he can't edit it anymore. How could I do this? I tried with rules but I can't get it. I could create another role and with roles changing group, but I'd prefer not to do in this way, so I thinked to a custom module, but I don't know how do it. Any suggestions? Thank you!!!

_{Edited by WorldFallz - moved to appropriate forum.}