There shouldn't be a need to delete it. If the processes in the process group no longer exist it will not be refreshed. Once this is outside the timeframe that you are looking at the process group will vanish in the UI.
What exactly are you trying to achieve?
I think my question belongs to here. I changed the Process group detection rules. I look forward to this change that more process group will be merged. But I still see the old autodetected ProcessGroups. They are not renamed as the ServicesNames used to do. How will that proceed?
the service naming rules, rename services. the process group detection rules change the identity of process groups, thus *new* process groups arise, which are not connected to the old ones. the old ones will simple age away and not receive any data anymore. no need to delete them.