"The decision remains the same – the deal, no-deal or no Brexit," she said at Prime Minister's Questions, BBC reported.
Labour leader Jeremy Corbyn accused her of having a "closed mind" to other Brexit options, such as Labour's plan for a customs union with the EU.
But the PM attacked him for refusing to meet her and said he did not "have a clue" what his own policy meant.
MPs are proposing alternative plans to the PM's deal with the EU, including seeking an extension to the UK's exit date – which is currently scheduled to happen at 2300 GMT on March 29.
But the prime minister has said the "right way" to rule out no-deal Brexit is to approve her withdrawal agreement.
Under current law, the UK will exit the EU on March 29, whether or not a deal has been struck. The decision to leave was taken by 52 percent to 48 percent in a referendum in June 2016.
British International Trade Secretary Liam Fox said delaying or canceling Brexit would be a "calamitous" breach of trust with the electorate and worse than leaving the EU with no deal.
He said MPs pushing for a delay actually wanted to stop Brexit – and they should think about the "political consequences" of that, not just the "short-term economic consequences".
"There is no doubt that leaving with a deal and minimizing disruption both to the UK and our EU trading partners is in our best interest," Secretary of State for International Development Penny Mordaunt told BBC Radio 4's Today program.
"But I think the most calamitous outcome would be for Parliament, having promised to respect the result of the referendum, to turn around and say it wouldn't."
But Conservative Remainer Anna Soubry said it was "not true" that Tory MPs backing a move to prevent a "no deal" Brexit – such as Nick Boles, Nicky Morgan and Sir Oliver Letwin – wanted to stop Brexit and had in fact voted for May's withdrawal deal.
Next Tuesday, MPs will get to vote on May's way forward on Brexit, after rejecting her initial plan by a record-breaking 230 votes.