As I mentioned in the early post: What additional powers does the Phoenix Force grant the “Phoenix Five”? The Phoenix is not in its ideal host. As a result, its capabilities are not nearly as omnipotent as they would be if they were within Jean Grey or Hope.
Their powers are a shadow of what they will be. They are currently utilizing their abilities at the peak of their mutant powers, but nowhere near the peak of the Phoenix's capabilities. It would likely be easier for them to destroy the Red Hulk than it would be to suppress his powers since none of their powers naturally suppress metahuman ability.
Think of the Phoenix Five as remotes for the Phoenix Force. Very clumsy remotes whose powers are nothing like the ones it is used to wielding. A threat like the Red Hulk is not to be trifled with and they would be best served eradicating it, if that is even possible.
It is also clear, they do not understand the mechanics of how his powers work, considering their bungled attempt to short circuit his powers using Emma Frost's psychic abilities. The Red Hulk was an unknown quality to the Phoenix and this worked in his favor as they used energy attacks against a being who only grew more powerful due to those attacks.