I have interpreted you what you in two ways. One you want to separate one tree into two tree, if so there are several threads on doing this on this form.

OR

You have placed a person in your tree that turned out not to be part of your family, and you want to delete that person and families connected through that person.

I was in this situation a couple of years ago. I had attached a parent to a grandfather that turned out to be wrong.

Instead of deleting the wrong parent and family, (I had spent years developing that family) I just unconnected the parent from the grandfather, and left the wrong parent and family as an unconnected tree in my database.

As it turns out it was fortunate that I did, as after several years work I was able to connect my grandfather to a brother of the original wrong parent. Fortunately I did NOT delete the family of the wrong parent, and had all that information in place when I attached my grandfather to the brother.

Because of my experience, I would recommend exporting the family you no longer want and maintain them in a separate file. OR do like I did just remove the connection and leave the family in your tree, as you may need it in the future.

In research all data is valid, you may not know what it means.