The management of security for large and complex environments still represents an open problem and the policy-based systems are certainly one of the most innovative and effective solution to this problem. The policy, that at low level is expressed by sets of rules, becomes crucial for the consistency of the systems to be protected and it is necessary to check it for correctness. This paper presents a set-based model of rules that permits the static conflict detection and an axiomatic model of conflict resolution leading to semi-lattices theory to solve inconsistencies. We proved the effectiveness of the theory implementing an extensible tool supporting security officers in creating rules by providing an easy environment to identify the conflicts and to use manual as well as automatic resolution strategies.