John the Baptist was positively not Elijah.

The Jews asked him if he was Elijah. Now
if anyone would know John's identity, it
would be John himself. He denied it.
John 1:21 "And they asked him, What
then: Art thou Elias? And he saith, I am
not. Art thou that prophet? And he answ-
ered, No."

This settles the fact that John was not
Elijah in person. But he did, come in the
'Spirit of Elijah'.