If you want to delete your family tree from your family site, please read the following article: How do I delete my tree from my family site?
In order to delete a tree from Family Tree Builder, go to File > Manage Projects. There you will see a list of the trees that you have created in Family Tree Builder. If you click on one of them, you can choose from the options on the right to delete it, and then click on ‘Done’ to finish.
Note: You will not be able to delete a tree that you currently have open in Family Tree Builder. Make sure to close the specific project that you wish to delete. To do so click File > Close Project. The program will still be open, and you will be able to delete your tree.