She can’t. The VP doesn’t have that kind of power. Never did. The VP “shall” count the electors.
Congress would have to refuse certification by disqualifying electors and therefore blocking them from being in the count. Not the VP. That’s the only way. And if neither candidate has 270 votes, the House elects the President and the Senate the Vice President.
So what I’m gathering from all these responses is that Mike Pence couldn’t have stopped the certification of Biden.