Suppose your "Don't Check" list is on Sheet2 in cells A1:A100
, say, and your current client IDs are in Sheet1 in Column A.
What you would do is:
Conditional Formatting
> New Rule
> Use a Formula to determine which cells to format
=ISNUMBER(MATCH($A1,Sheet2!$A$1:$A$100,0))
and select how you want those rows formattedAnd that should do the trick.