You can't delete the branch, but you can disconnect from it. Open your profile, and go to the Edit screen. On the right, you can mark your not-father as non-biological, but he'll still show as your father in the tree. Or you can click Edit Father, then click Remove. Either way, you should probably add a note in your profile regarding the DNA results.
And welcome to G2G. Sorry if this discovery was traumatic for you.