This paper presents a control synthesis approach for discrete event systems modelled by partially observable marked graphs. It solves the forbidden state problem for discrete event systems modelled by partially observable event graphs subject to Marking Exclusion Constraints (MEC). Two kind of MEC specifications are considered: MEC-AND or MEC-OR. The main advantage of MEC specifications is an increased modelling power regarding to General Mutual Exclusion Constraints GMEC. We provide a simple optimal controller to enforce MEC specifications with independent critical places. We also propose a suboptimal controller for MEC-OR with dependent critical places belonging to the same direct path.