Devices are still polled during 'planned downtime'. It's only for device uptime report that 'planned downtime' makes a significant difference. I don't have a pretty way to do what you're trying to do, but try this:
Go to Group -> select group -> Basic tab
Disable polling, save and apply, apply
Go back to Group Basic
Enable polling, save and apply, instead of apply -> schedule for repeating option
Go back to Group Basic
Enable polling, save and apply, apply
Go back to Group Basic
Disable polling, save and apply, instead of apply -> schedule for repeating option
This should do what you're trying to do. While at the same time keeping a planned downtime window for the time that's not polled in case you want to run your device uptime report.