I thought that I could use a dhcp server on a raspberry pi to solve this problem. However, I ran into a HUGH reason there is no white list. So, here is my solution. It is not perfect because because phone makers and maybe computers as well have gone to randomizing Mac Addresses. This means that every time the user logs in it will be with a different mac address. This is to protect our privacy. If a vendor saves your mac address it does them no good because they cannot connect it with the other purchases you make because you are logging in with a different mac address every time, unless your phone is very old. 

So, here is my solution. I have a limited number of known userers and I want to keep everyone else out. 

1. In your deco device on the opening page on the bottom right click more.

2. Click advanced.

3. Go to address reservation

4 Click the + button top right 

5 Select Custom

6 Choose a block of ip addresses in the middle of the range from 192.168.68.100 to 192.168.68.250. The static ip address you want to give that user and the mac address from the users phone mac address.  

7. Set the IP address in the user's device for their connection to your network to the static ip address that you have saved in the Deco device. You can leave the mac address type at randomized or change it to phone mac address. It is working either way for me. 

8. When all of the ip addresses are set up go to your deco device again and above address reservation you will see DHCP server. Change the range of the server to match the range of the addresses that you entered. 

Drawbacks: If the user, you don't want to come in, figures out your range of static ip addresses they can change their phone and keep selecting static ip addresses until he hits one that is not modified. This may not work for them if you make your users use only the phone mac address. I have not tested it. I have tested most of this at home and will implement it soon in a little larger location. 

Good luck!