I have been telling people to NEVER import an unknown GEDCOM into their main database.Always import it into a new and empty database and then extract, by GEDCOM exporting a few at a time, only the families you really want in the main database. But some people don't get a chance to hear my preaching. Your best bet would be to create a new database and restore a recent backup. Failing that, the next thing is to export the wanted family trees from your main database and import them into a new database. This new database then becomes your main one and after some time, when you are positive you want no more of the people from the original, you can delete it. You can't selectively delete families, but you can selectively export them.