@ budowski,
If you are extending 2.4Ghz, then, tap on 5Ghz and change the security from unsecured to WPA/WPA2. However, we would not be able to make any security changes to the network (band) to which the Almond 3 is connected and it would still be open.
On the other hand, you would have the ability to find out what all network devices are connected to Almond 3 and then block/allow it accordingly. Thank you!