If one or more effects say you "can" do something and one or more effects say you "can't" do that same thing, the "can't" effects always take precedence. For example, explicitly allows you to untap an allied minion, while prohibits other minions from untapping. If Queen Guinevere tries to untap any minion other than Rhitta, the effect will fail because the "can't" effect from Rhitta takes precedence.
Note that "lose" effects (e.g. ) also implicitly include a "can't" effect. So if Sky Baron is in the realm and then you cast , the "can't" effect takes precedence and the minion doesn't gain Airborne.