So B would be a
subset of A, you are looking to subtract B from A. This is A-B = {aϵA: a ¬ϵ B} where ϵ is the membership and ¬ is negation. In words this would mean that the set A minus the set B is a set of all the elements of A that are not in the set B.
If you need help with how to specifically do that one would need to know the file format the file is received in. Ideally you would receive a csv file where ids are separated by a newline or a comma, in which case you can use whatever tool you want. Python or excel for example
Python:
A = open("A.csv",'r')
B = open("B.csv",'r')
C = []
for element in A:
if element in B:
C.append(a)
If it's excel files you are receiving, using excel will be easier, it's more user friendly than a programming language. In this case you need to know how to use formulas, you would just need to know how to check for membership, how to negate a boolean, and perhaps how to use a conditional. I believe these are the Match function, the Not function and the IF function respectively. The end result should be something similar to the python code above, the mathematical formula, and the english explanation. Find someone in your team who knows how to use excel, or take a 101 course on it, should be no problem.