BleepingComputer.com: I have 2 lists of sites. How can I use one to remove the sites from the other ?

For example:
List A 20 000 sites.
List B 9000.

I want to use list B to remove the the sites in List A .
The sites in List B are all in List A so I want to remove them.

You could use Excel's VLOOKUP() function IF and ONLY IF the format of both sites is identical in both.
Put List A is in column A. List B in column B.
You'd need to sort list A, the bigger one, on, let's say, site name.
Then in Column C you're write an expression to lookup what's in B in column A. If true, it'll list a matching value from column A, if not found it'll say "#N/A"
=VLOOKUP(B1,A:A,1,FALSE) - make sure False is at the end for exact match. "1" stands for column to match. If your list has more columns than one, tweak the formula.
Then in Column C setup auto filter. Select all the non "#N/A" and delete in A since those are in both lists.
Once caveat, I think excel will only permit you to use 16345 rows, approximately, in which case you'll need to work with two worksheets.

Really, what are these "sites" about? Perhaps the applications that make those lists can eliminate the duplicates. What if you merge the two lists?

Access can do similar thing for you with a query for common items. Read Help.