Recently i noticed that there is an unknown user connected to my modem. I tried to block the mac address by accessing router page(192.168.1.1 ,dlink modem) > parent control > macaddress filter > put the mac address of (unknown user) > apply. but they are still using my wifi. This leads to slower internet . even youtube is slow now. So what should i do? google search lead to this link <https://security.stackexchange.com/a/137316/149263> and after reading this, i am sure that someone is stealing my wifi. i just want to block the user . is there any way to block the user permenently? 

there is also one thing to notice that the mac address of unknown user is similar to my device (only 3/6 parts are different). if **01:02:03**:04:05:06 is my mac address then the mac addr of unknown user is **01:02:03**:0a:bb:cc. i will provide screenshots if required. sorry if its offtopic here.