No matter WHAT evidence anyone comes up with,AFAIK there is NO way to nullify an election. Once a President has been sworn in it is a done deal and the only "remedy" is to make him as powerless as possible,and vote him out come the next election.
Yeah,it sucks,but it is what it is. Anything else would be an admission of failure by our system,and that just can NOT be allowed by the politicians on either side of the aisle.