@Pedro
Unfortunately, as mentioned above, we cannot support multiple 'enabled' periods in a single day yet. However, just note that we are currently working on enhancements to the sliders on various pages of Dashboard. The cameras, for example, have some extra scheduling options in defining when they record throughout the day. It hopefully won't be too long before we see similar functionality with the SSID scheduling!
One workaround could be to setup custom group policies that are based on the time you don't want people to have access to the SSID. If you block all internet access using L3/L7 firewall rules during that time, then apply that policy directly to users, you can achieve similar results.