It shouldn't use a full Corona license, just a render node - assuming you have a separate render node that is? If you don't have a separate render node license, then indeed the render node part of the full license will be in use.
Also, a full license will be consumed if you open Max or C4D on the render node though, but as a render node you don't need to do that (just run Backburner Server on it, rather than opening Max). Simply closing Max should free the license in that case, and indeed stopping rendering and closing the backburner client should also free the license.
Let us know more details on the situation, ie do you have a separate render node, etc.