The detection hiding options shown are based on the particular detection you clicked to hide. For the offender and victim dropdowns, this means that only device groups that contain the offender or victim are shown, as these are the groups that directly relate to that detection.
When the offender or victim is an IP address that is not associated with a discovered device (as is typically the case with an external IP address) it is not part of any device groups. In this case, the only options are to hide detections in which the victim is that specific IP address or any device at all.
Is there a specific device group you want to ignore as victims in for a certain detection type? If so, you can do this by clicking the Tune button on another one of these detections where the victim is a discovered device in your environment.