The only way to prevent from distrupting the local viewer is to tune into a video source which isn't being viewed locally. This can be achieved with connecting your current Slingbox to another cablebox, or using Slingbox Pro-HD's internal tuner.
The Slingbox Pro-HD has a hybrid analog and digital tuner which is capable of tunning in channels from over the air ATSC or unencrypted QAM signals. If your television provider broadcast any channels on a compatible signal then it would be able to pick it up. Since you'll be using the internal tuner, then you won't be interferring with the local TV.