how to block devices on U-verse gateway settings

You should find what you're looking for under Home Network > MAC Filtering.

Where do I find MAC Filtering? I don't see it.

Where exactly are you looking? 

Im in my Gateway settings and I've been looking all day for the option to restrict wifi access to a device.

I’ve got an old gateway, so not sure if my interface looks the same but from the home page, select wireless. On the resulting page scroll to the bottom, there is a link for MAC filtering. This is my shortcut to that page, it might work for you: http://192.168.1.254/xslt?PAGE=C_2_1a

I'm working from a BGW 210, but a BGW 320 should be the same.  As soon as I get to the gateway, I just click on Home Network, and then the MAC Filtering subtab.

The PACE-based software would have links like sandblaster's, but I haven't had one of those in a while and I don't remember what the pages look like.

Is there a way you could give me a rundown of how it works?

Reference the screenshot below.  You can choose for each of the three Wi-Fi networks ( Guest[2.4 MHz], 2.4 MHz and 5.0 Mhz) whether to enable a MAC whitelist (Allow only the list) or blacklist (Deny MACs in the list).   This is at the top of the screen.

Then, below, you can add MAC addresses and select which of the above network lists this MAC address should appear in.

So you could decide to block Mac 01:02:33:40:55:FF's access to the 5.0 network by setting 5.0 to Deny and adding that MAC to the list and checking the 5.0 box.

So i'm on the MAC filtering, but the device i tried to block wasnt affected