They can say what they like but what they can't do is try to reverse it. Which is what their version of right is.
Of course people can try to reverse it - they have every right to do that.
For ordinary members of the public the only way to do that is to persuade sufficient people to use democratic means to reverse it - either through persuasion that a second referendum is a good idea or through a general election which returns a government with a mandate to reverse it.
For the current parliament they have every right to reverse the decision - as parliament is sovereign (remember that's what you brexiteers were fighting for) and can chose to ignore a advisory referendum.
In reality the most likely route is if the deal eventually struck turns out to be a dogs dinner (as is likely) and clearly not in the best interests of the UK, resulting in a clamour to revoke article 50, with or without a further referendum.
However the major point remains - we have every right to try to reverse the referendum decision. Otherwise we'd never be able to change our minds in a democratic system, i.e. we could never kick out a government that we'd previously voted in.