In the digital age, individuals increasingly maintain active presences across multiple platforms ranging from social media and messaging applications to professional and communication tools. However, the current model for managing user level privacy and abuse is siloed, requiring users to block undesirable contacts independently on each platform. This paper introduces Single Block On (SBO) a unified and interoperable system enabling users to block an individual once and have that block propagated across all integrated applications. SBO operates via identity based matching rules, utilizing configurable levels of identifier similarity, and interfaces with systems through standardized protocols such as SSO, LDAP, or direct REST integration. A novel Contact Rule Markup Language (CRML) facilitates consistent policy sharing across systems. The proposed solution increases user safety, enhances digital well-being, and sets a precedent for interoperable privacy enforcement.