Hi
1. Is it the correct way to release moment Mx if I do not want to design the beam to take torsion?
Though it is unlikely that a RC beam is allowed to rotate about its own axis or cannot resist torsional moments but if you want to simulate the same in analysis, releasing MX is the correct method.
On the other hand, if your intention is to ignore torsional moment while designing, TORSION parameter exist in some design code for the same purpose. In Concrete Design Mode (RC Designer), you will find a check box in Design Briefs to design for torsion or not.
2. Will the release of Mx cause the moment to be transfer to the support?
Releasing MX would mean that the member is free to rotate about its own axis and this will definitely affect member forces which are calculated on the basis of displacement results. In short, member forces are going to be redistributed.