@SCampisi- You don't need to delete or change the default rule. The rules are processed top down, so when you add a new rule for the SSID and set it to Deny Any Protocol to Any Destination that will be the top rule.
At this point, your clients would be able to connect, but not access anything.
Then you could go to Network Wide/Clients, click the check box for the clients you want to allow, then drop down the Policy box and add them to the Allow list.
I just tested this in the lab here and it seemed to work as expected.