It is practically impossible to teach good programming to students that have had a prior exposure to BASIC: as potential programmers they are mentally mutilated beyond hope of regeneration.

This quote means that it is very difficult to teach students good programming if they have already had some experience with the programming language BASIC. BASIC is an older programming language and is considered to be of lower quality and difficulty than more modern languages. The quote states that if a student has already learned BASIC, they have been “mentally mutilated” meaning that they may have bad habits which cannot be unlearned or fixed. As a result, it is practically impossible to teach these students good programming.