I think before I completely deleted all of the people, I would export him and the his associated generations to a new FTM file under a different name. Once I had the people exported correctly I would then delete the line form your tree as mentioned above.
Genealogy runs in mysterious ways. You never know when there will be a tie back into this family. For years they were similar enough to your family to think they were yours.
I had a similar situation. I had worked on a person and his family for about 7 years, and developed good documentation on both his ancestors and descendants. Then I found the document I was looking for and it completely eliminated the person as a direct ancestor.
Fortunately, I did not remove the person and his family from my database, as several years later I did find the direct ancestor and he was a brother to the person I had spent 7 years researching. So All I had to do was connect to a different brother and I had the family for several generations back.