How to withdraw a paper (under review) from fake journal
Simply let the editors know that you withdraw your submission. Tell them that you do not grant them any right whatsoever to internally keep or externally distribute that paper.
Publishing occurs on the basis of a bilateral contract. If one of the parties – in this case: you – does not agree with the agreement's core performance (here: the publication), there is no contract. No obligation arose for you.
Things, however, may get complicated: It is possible that the publisher will claim that a contract was initiated through your submission (and with the help of some hidden agreements and terms of services). In this case, you can argue that entering into that agreement occured &quot;under a mistake as to a matter of fact essential to the agreement&quot;. Make sure to list any evidence that would qualify that publisher as predatory, that is, any evidence through which you were misled about their identity. In many laws around the world, there are explicit provisions about contracts being voidable if it was agreed upon under an erroneous belief about the identity of the contract partner.
But let's hope that no argument will arise. Otherwise it would help to know which publisher you are talking about, and which country's law is applicable in your case.