Andy is very convincing. Andy is very persuasive. Andy is a good salesperson. Andy "can talk you into anything." Nick wants to say no to Andy. Nick knows that he should not agree with Andy. But Nick has never met anyone as convincing as Andy.

It can get confusing when English phrases/sentences end with a preposition, like in 'impossible to say no to'. Sometimes it is easier to understand (for non-native speakers) to put the preposition at the front, although this sounds more formal. With this example, we could also say: "someone to whom it is as impossible to say no" (this is the more formal version of "someone as impossible to say no to". Whichever way you choose to say it, it's very important that you keep both of the to's (the 'to' that tells us it is an infinitive verb, and the 'to' that is the preposition ending of the phrasal verb telling us who is 'receiving' the no.).

It means he never expected to meet someone like Andy to whom he was unable to say: "No, you cannot come into my life".

It means that Andy is the kind of person who cannot accept “no” for an answer. Andy is a very persistant person. This is usually a negative trait.