There are ways in RM to "select a person and their ancestors", but such ways don't work for deleting people. By design, RM does not support any kind of bulk delete. Instead, the largest collection of people you can delete from RM all at one go is a family unit - mother, father, and children. RM has stated that they will never support a bulk delete. The concern is that a user will bulk delete by accident and not have a backup. And remember that RM does not have an undo capability. The way you do an undo in RM is to restore from a backup.
The workaround is to drag and drop an existing RM database into a new and empty RM database. As a part of the drag and drop process, you can mark people to drag and top to the the new and empty database. Anybody not so marked is not copied to the new and empty database. The marking process for drag and drop does support bulk marking - like all ancestors, etc. Also, you can initially mark everybody to keep, but before you complete the drag and drop you can unmark those you don't want to keep. This is more like what you want to do. So you could drag and drop, mark everybody, and then you can bulk unmark those you don't want to keep.
There are certain data items that can be lost on a drag and drop. I'm never quite sure how big of a deal to make about such data losses. Most of them are fairly obscure and probably won't affect most users most of the time. But they do exist and they can be concerning.