Edge rerolls on the summoning test is a pretty big influence on how powerful a summoned spirit is going to be. It's legal for SRM GMs to have spirits spend edge on their own behalf to resist the summoning, but it's also somewhat antagonistic on the meta level. There's definitely a dimension, or at least the perception, of a GM going "I don't want you to have this..."
If you open up the possibility of house rules/deviating from the rules, you could flip that equation on its ear and just forbid ANY rerolls on summoning. That'll result in spirits with less services, and less services equals less potential to be problematic for game balance.
Another option would be to reject the standard of 1 combat = 1 service. If 1 combat round = 1 service, or any 1 use of a power = 1 service, you get to the same place as above, but without taking away the player's right to spend edge on an unsatisfactory roll.
Barring any of that, about the only thing you can do is exercise GM authority to tell the player what the spirit does, rather than the other way around. And, if there's a game balance problem, the spirit can do things 'according to the letter of the command' but twists the intent in such a way to not provide an "I win" button to the problem-maker.
Edit: you could also play around with nerfing Immunity to Normal Weapons, but in 5e it's less of a problem. But if it IS a problem, remember that ItNW is tied to essence, not force. There's no reason why a force 6 spirit has to come with full essence. Maybe it's only got 2 or 3 essence, because it was summoned in a polluted sprawl... And of course you could just directly nerf the effect, so that the kinds of cheap weapons employed by mundane opposition could plausibly hurt a spirit.