Anybody still read/watch Katekyoo Hitman Reborn?

I don't think it's that he can only be stopped in the future, it's that he's holding them there.

Once they defeat him in the future, they head back to the past and defeat (or befriend) a presumably weaker Byakuran.

Of course, his past self probably knows what will happen, due to a potential future where he has seen what will happen...
Time-related powers get incredibly confusing...