I'm looking for a function/formula that would exclude zipcodes in Report A's output if they're not in Report B.
You can achieve this via the Join Tool, but make sure to take the unique Zip Codes from Report B - otherwise, the records from Report A might duplicate because I can already see duplicate Zip Codes in your Report B.
To do this, I would use the Summarize Tool to Group By the Zip Codes from Report B before performing the Join with Report A, like in this screenshot: