Millimetre-wave ultra-dense high capacity networks are an important component of future 5G and 6G cellular systems since they are providing extremely high network capacity and speed to the end users. However, disparate types of users coexist in such scenarios, which can make the heterogeneous network unfair in terms of allocation of resources to various users based on their requirements. Therefore, a mechanism is required for effective spectrum sharing and to achieve overall system fairness. In this paper, an analytical model is suggested, which is based on a two-dimensional Markov state-transition diagram, to help set the parameter values to control the issuance of resources in coexistence layouts. A restriction approach is further implemented to gain a fair balance of the Grade-of-Service (GoS) for both user groups using the User Admission Control (UAC) mechanism. The developed mechanism restricts access to various channel resources for users with complete choice to give a greater probability of access to different users with limited resource options. Various levels of restriction are investigated in order to offer a balanced low-blocking probability performance to both user groups in order to improve the overall network fairness. Also, the proposed approach could provide a precise level of Grade-of-Service guarantee for both the user groups if sufficient flexibility is available within the whole network. Our simulations results along with the analytical model demonstrate that approximately 30% to 45% enhancement, in terms of grade of service (GoS), could be achieved in high to medium loads by restricting some of the users’ flexibility. From the analytical model, it is concluded that the blocking of shadowed users are significantly reduced from 7% to 4.5% at high traffic loads. Moreover, the obtained results and findings are verified using a number of case studies and large-scale simulations.