Have the judge fine them for neglecting military duty then. This is not a problem which needs to be addressed OOC. In fact, it is not even common. It's only on the forum that I occasionally hear stories of how Aurvandil had really high tax rates, or how Allison ran her city at the maximum tax rate. I've never actually seen any of this in the game, all of my lieges invariably set annoyingly low tax rates, far below what could be supported.
what is your idea of normal taxes?