You can use the 'Security > MAC Address Table > Static MAC' security feature, where you disable 'auto learning' of MAC addresses for any port that is not in use (uncheck these ports in the list of 'Uplink ports'). In addition use 'Security > Port Security' where you specify the range of ports your 5 computers are connected to (should be a continuous range without unused ports in between), for which you set the 'Admin State' to 'Enabled' and specify a 'Max Learning Addess' value of '1'.