Substrate ion fluxes from unbalanced magnetron sources are generally controlled by the use of additional magnetic fields. These methods are not always suitable for differentially controlling the ionic flux from multiple sources. We report here a simple method suitable for attaining such control that can be easily adapted to most unbalanced magnetron systems. The flux leakage from an unbalanced magnetron is controlled by the use of a mild steel keeper with a detachable mild steel annulus. The annulus redirects the unbalanced field lines through the keeper to the rear poles of the magnetron. The flux leakage is thus controlled by varying the thickness of the annulus. Ion saturation currents measured at the substrate, with and without a 0.9 mm thick mild steel annulus, were found to differ by a factor of approximately 3 at an argon partial pressure of 1 mTorr. Furthermore we demonstrate that the changes in ion current density are achieved without alteration to the magnetron operating parameters.