System method and program for controlling access to a VLAN via a port of a VLAN switch system. In response to receipt of a message packet at the port, the switch system determines if a MAC address of the packet matches a MAC address for which the port has been programmed to recognize as a MAC address of a device authorized to communicate with the port. The MAC address of the packet does not match a MAC address for which the port has been programmed to recognize as a MAC address of a device authorized to communicate with the port. In response, the switch system blocks the packet if a rate of ill-formed packets and/or packets from an unrecognized MAC address exceeds a threshold pass rate. The threshold pass rate can be adjusted based on the rate of change of receipt of ill-formed packets and/or packets from an unrecognized MAC address.