So, this question is old - but I'm still clearly having this problem.
I want to block 1 device from my wifi;
I go to mac filtering, add the device and set a rule - Apply;
Then all my devices are blocked from the internet!!!
There is a previous (complicated) workaround from a user/customer using port filtering etc but it is too complex for me to try;
Why does the filtering not work? and how do I simply block 1 device?
I recall there was previous mention on the forum of making sure Advanced Settings > Security > Wireless MAC filtering option was set to Deny.