I see no one has replied.
As this is a GRE scanner, it appears what you are asking is how to unassign all objects from a specific scanlist?
In the old days some scanners allowed entire 'banks' of 'channels' to be erased/deleted. However, because GRE scanners use 'objects' that can be assigned to 'multiple' scanlists, we are dealing with something different. There is no way that I am aware of, using only the scanner, that a person can unassign or erase/delete all talkgroups from a specific scanlist other than one object at a time.
Software programs can accomplish what you are requesting. I use the Win500 program. In the Win500 program simply highlight all objects in the scanlist and then click on the scanlist number you no longer want the objects to be part of and they are all removed from that scanlist or press Alt + Delete and all the highlighted items are erased/deleted entirely from the scanner.
A software program sure saves time in many situations with these newer scanners!