Sessions can’t legally bar them from testifying.
Sessions is providing them cover. He knows any testimony sinks Comey, et al. Bigger problem. Clintons for years have played the Congress angle and now they are doing it while Comey’s back is against the wall. Comey would have to take the fifth and so would many of his buddies. This sets up a situation where lawyers get to debate whether any prior testimony is valid. Sessions covering all the bases to let Comey and hillary off Scot free.
That's true, but he can tell them that their plea bargain deal is null and void if they talk before he indicts the big fish.
Of course, that assumes Sessions is on our side, which is getting harder to believe by the day.