We have a need to stop all data aging for our NDMP so nothing else gets deleted. Is this as simple as going to the storage policy copy and unticking the Enable Data Aging? Would this keep all our current jobs or will it just stop aging from new jobs created?
Yes; Disable DataAging by unchecking "Enable DataAging" option on copy.
DataAging job will not prune any jobs on that copy until the option is enabled.
If there are more clients related to that storage policy your better of by disabling data aging on the specific NDMP clients, this is located on the Activity Control tab.
Another option is to granularly marking each job by using the Retail Until feature without any broader impact, this way you target only the jobs you want.