The contract is tripartite with the player, the rebels and RA and technically it has already been breached as it is to play super rugby in Melbourne. Part of that (or maybe part of the CBA separately, I'm not sure of the structure) is that RA has agreed to honour all contracts in the event of a team being removed from super rugby, and that they can't direct players to a specific team, or any team at all. These players seeing out their time in club rugby is honouring the contract they, and RA, signed.
I would be amazed if RA took the never to return stance - it would be cutting off their nose to spite their face and totally out of line with their handling of Izack Rodda's departure from the Reds and return to the Force, which is another situation of a player not agreeing to a variation. Similarly what would be the difference between these players and Uelese and Kemeny who have already departed?
Also, I'm not so sure that sort of blackballing is entirely legal and RA might get themselves into more trouble voicing that stance.