The Welsh actor was accused of pushing the guard after she refused to let a member of his entourage attend his panel for The Amazing Spider-Man at last month's conference.
She performed a citizen's arrest on the grounds of misdemeanour battery when the Q&A session concluded.
However, the office for the San Diego City Attorney has now confirmed that it will not investigate Ifans's actions at Comic-Con any further.
"After reviewing differing witness accounts, our prosecutors concluded there was not sufficient evidence to prove the case beyond a reasonable doubt," a spokesperson told The Wrap.
A spokesperson for Sony has since claimed that the 44-year-old "deeply regrets the incident".
Ifans declared last month that his Amazing Spider-Man character Dr Curt Connors, aka The Lizard, is not "necessarily a villain".
The Amazing Spider-Man is due out in US and UK cinemas on July 3, 2012.
Watch the trailer for The Amazing Spider-Man below: