Device still on my wifi with Mac filter in place
So, never tested this before, but decided I would give it a go, and detail what I have tested and found.
1: I have never connected my phone to my HT2000w Wifi, so, I enabled the Mac Address Filtering and set it to Deny the MAC tied to my phone. I could see the network, but would get an authentication error immediately when trying to connect.
2: I disabled the filtering, and have connected my phone, then verified that the HT2000w sees my phone and that it's in the listed devices section of the Wifi Settings section. I have also verified the device is listed under the Advanced page settings of the HT2000w.
Once that has been verified, I went and enabled the Mac Filtering with a Deny rule for the MAC address on my phone. My phone after a few minutes immediately disconnected. I then checked the two locations where devices will show up.
In the Wifi Setting page, it no longer showed the phone at all, but in the Advanced Configuration page, it now shows my phone as disconnected.
I have also verified on my phone, that I am not able to connect to my HT2000w anymore.I think what has happened is that the person has multiple devices, or is spoofing their MAC address. You would be best off to in the future enable the Guest Network, and use a unique password on that, that you can freely change with no worries at any time. The best course of action right now, is to change your primary network passwords for both the 2.4Ghz and 5.0Ghz bands, and if you have guest networks enabled, change those passwords.