I want to delete just part of my least engaged contacts

Hello @RobertScottBell ,

The segment with the lowest level of engagement is presented as is, rather than a ranking of contacts from most to least engaged. If you have particular criteria you'd rather measure your "least engaged" by, you're welcome to create a custom segment with relevant criteria (.e.g. Activity > Not Opened > All emails in a range of time). 

You can then further manage this segment by adding it to a list, determining which contacts may still be worth keeping (such as very recent additions based on Date Added), and then deleting all the remaining non-openers and the segment list.