Pick one or two.
Charge them with something,fire them and let that play out, right or wrong.F’em the others will fall in line
Firing is not a remedy.
Indictment and jail for a few days during the arrest process is the least that is necessary. There were laws broken that must be enforced up to and including death as punishment.
Or the really guilty ones might think they got away with it, the mole hunt is over, and they continue leaking.
The smart way to do it would be to create a hundred different versions of every document distributed, each one with minor differences in font, punctuation, spacing between words or sentences or paragraphs, and keep track of who got each copy. Then you know who leaked a document by which version surfaces. Phone calls are tougher and require warrants to track private phone usage. You could ask people to voluntarily give their phone records, and then exile any who refuse.