In the service oriented architecture framework, a modal specification, as defined by Larsen in \cite{Lar89}, formalises how a service should interact with its environment. More precisely, a modal specification determines the events that the server may or must allow at each stage in an interactive session. Therefore, techniques to enforce a modal specification on a system would be useful for practical applications. In this paper, we investigate the adaptation of the supervisory control theory of Ramadge and Wonham to enforce a modal specification (with final states marking the ends of the sessions) on a system modelled by a finite LTS. We prove that there exists at most one most permissive solution to this control problem. We also prove that this solution is regular and we present an algorithm for the effective computation of the corresponding controller