I am preparing for the grand finale by re-reading Book 6, and the last 100 pages of Book 5. At the end of Book 5 (Order of the Phoenix), Dumbledore reveals why he left Harry with those awful Dursleys. It was love, something Voldermort doesn’t appreciate. Aunt Petunia, however unwillingly took him in, and because she is the same blood of Harry and his mother, that made the house a safe haven.
Okay, fine. So you can’t get at Harry while he calls it home. Maybe you can do something about that. Nothing says you can’t enchant the Dursleys. With them under your control, how hard could it be to get Harry to renounce his home? Drive him to move out, or go back to Hogwarts. Then you ambush him on the road. Not terribly hard when you’re the Dark Lord. Don’t try to kill him, but get your Death Eater pals to capture him.
Even that is over-complicated. Just get the police to arrest him, and have them bring him right to your place instead of a normal cell.
Then you just throw him in a nice quiet cell with no means of escape or communication and wait for him to starve to death. You don’t have to worry about the scar or the prophecy or anything like that since you’re not actually attacking him. As long as you’ve covered your tracks well enough, he just dies.
Honestly, how hard is this stuff?