Web-based data collection of non-reactive data is becoming increasingly important for many social science fields. Being able to introduce and automatically enforce policies that regulate the collection and the use of those data is crucial for taking into account the privacy and confidentiality wishes of data providers. Those policies are currently expressed in natural language or in a language lacking a formal semantics, therefore, in order to comply with them researchers need to apply them manually. It is therefore clear that realizing new technologies for automatically supporting the enforcement and monitoring of various type of policies in Web-based data collection is an important open issue. In this paper, we propose an application independent formal model of policies that can be automatically enforced and whose specification allows to take into account the meaning of the managed data. Given that for enforcing the policies on the collected data, their semantics is crucial, in this paper we propose an OWL 2 Social Network Ontology to capture the nature of social network data collected from the Web. We also propose techniques for semantic enrichment of the data, which are then exploited in the specification and enforcement of the formalized policies.