Unfortunately, I don't think there is a way to remove a branch on a family tree (if you don't want to delete each member of the unwanted family individually). If you are just annoyed by seeing them, maybe it could be an option to visit the "Options" setting of your family tree and select the number of generations displayed on said tree? You can select how man generations of parents, children and other ("other" being aunts/uncles, their children, their partners, their partner's relatives and so on) you want to display - if you select "none", you'll only see a person's direct ancestors, siblings and direct descendants.
Depending on how big your family tree is, how many branches it has and what you want to use it for, this option might not work well for you, as you might want aunts/uncles and so on to be displayed.
Hope this helps!