In previous work, we have proposed a fully probabilistic version of Event-B where all the non-deterministic choices are replaced by probabilistic ones and, particularly, the events are equipped with weights that allow us to consider their enabling probability. In this work, we focus on the reliability of the system by proposing to constraint the probability of enabling an event (or a set of events) to control its importance with regard to the intended system behaviour. We add a specific upper bound which must limit the enabling probabilities of the chosen events and we consider the necessary proof obligations to check that the considered events respect the bound. At the end, we illustrate our work by presenting a case study specified in probabilistic Event-B and where bounding the enabling of some events is mandatory.