I would leave it up to your boys as to when to see her again. They are the ones who deserve a relationship with her, if they want it. So if they want to go see grandma next week, although that doesn't sound likely, but if they do want to, then they should be allowed. On the other, more likely hand, if they don't want to see her for a year or two, well, that should be up to them as well.
But I do completely agree that when/if they ever choose to see her again, it should be supervised. The fact that she is so sure she is right to the point of not even offering any sort of apology even AFTER the fact also disturbs me a great deal. I would be wondering what else in the past she may have done against your wishes, perhaps in terms of food, discipline, etc.
And whatever you do decide, no matter how livid you are, put your parent hat on & calmly explain to her that she now has to deal with the consequences of her improper behavior & actions, for two reasons. One, because it it the proper, mature way to handle the situation, and two because it will throw her off, she'll be less likely to know how to repond. She is expecting the anger, and is obviously all geared up to respond to it. Or better yet, let the kids tell her what her consequences will be (with you present of course, or listening in if it's on the phone). Ask them what they think would be a fair consequence (I hate to use the word punishment, even if it is kinda appropriate this time) for grandma's actions. Since she showed no regard for their opinions when she forced her opinions on hair length upon them, I think it is perfectly fair to reverse the situation.