An access control system (200) enables a computer network (1) to prevent execution of computer code that may contain computer viruses. An access control console (201) generates an access control message (260) including control parameters such as a time limit (255). Said time limit (255) is disseminated to computers (2, 3) on the network (1). Said computers (2, 3) use the time limit (255) to determine the executability of computer code. Access control system (200) also enables blocking data communications with suspicious or susceptible programs in network (1) during virus outbreaks.