The bigger question is why you'd even want Ezra Miller to sign something Marvel-related when he's playing The Flash in the DC Cinematic Universe.
In any case, Ezra Miller isn't signing or touching anything Marvel-related and this video from the London premiere of Fantastic Beasts And Where To Find Them just proves it, to be honest. When given an Agents Of SHIELD poster to sign, Ezra looked somewhat perplexed and then admitted he wasn't allowed to sign it.
"You understand why I can't sign that, right?", said Miller, who then offered to sign literally anything else that wasn't Marvel-related before moving on. This is almost like but not bad as that time Jack Nicholson was confronted with a picture of Heath Ledger's Joker and asked to sign.