ALWAYS create a backup before doing this, as there is no other way to get the deleted records back if you make a mistake.
You select the fimily lines and individuals as if you were going to Export them as a gedcom file, but instead of clicking OK, you click on Delete.
You can use this sequence: Search > Advanced Focus Filter > (use Relationship filters and select the lines and people to delete ... they will be marked by a >> to the left of the name) > Delete > (and follow through)
It is very important that you check everyone marked with a >> to make sure you want to delete them. If you don't want to delete them, you can click on Clear to clear the selections you have made. Or with a little practice you can use Deselect to remove people from the list of those currently selected. If you accidentally delete people you wanted to keep, restore the backup.
(I give each backup a different name so that it doesn't replace the previous backup.)