I agree with the majority opinion--you have to decide with your husband how your household is going to be run. This is not acceptable--your stepdaughter is setting the rules, not her father, mother, grandmother, or you. It's totally backwards! It's not desirable or feasible for you never to be able to discipline your stepdaughter in the absence of her parents; but you do need to be on precisely the same page with your husband, so that he can back you up when the ex-wife or his mother calls. He needs to support you 100% and tell them to stop calling him....but to do that, he must be able to support you 100%, and you may not like having to change how you're handling the situation in order for him to support you. But the trade-off will be worth it. You are not this little girl's mom, but you should have authority over her, as an adult and her step-mom. However, if your husband is not going to support your decisions, then he needs to tell you which decisions he will support. I also think it would be a great idea for his ex-wife and mom to be involved, so that they can give ideas of what is to be done when the girl behaves inappropriately. This can be constructive, and should be done without her presence. It may be preferable for the four of you to go out to dinner, and have a "round-table" discussion, where you can present your problems--be honest, but be sweet. Express your concerns about your step-daughter's behavior, and fears that she will get worse if you don't do something; and ask them what their opinion is when X happens. Give them a real situation that has happened recently--how this girl did something, and ask if that behavior is appropriate? If they say "yes," then you'll have to alter your ideals for this poor child; if they say "no," then ask them how they would deal with the situation. If they would deal with the situation differently, then be prepared to change how you handle the matter; if they say you need to ground her from the computer (which is exactly what you did), then tell them that's what happened.
Your step-daughter is driving a wedge between you and your husband, and is playing her parents off of each other, and getting her grandmother involved, to boot. This is normal, but not acceptable. You are adults, and need to be able to handle this matter appropriately. Once the four of you are in agreement with how a situation should be handled, then she will not be able to start this power struggle.
Your husband may need to just tell his mom that it is none of her business, and she is not to call him any more when his daughter complains about you. And he should encourage his mom not to listen to such complaints; and tell his daughter not to call her any more.
The ex-wife is a little trickier, because this girl is her daughter too. If you all get on the same page, then that should handle the situation. Be prepared to swallow your pride (and possibly some of your ideals) so that you can follow the ex-wife's rules on handling her daughter. It will not be fun, but it will be worth it in the long run.
Your husband is the central player in all of this--he must make the decisions, and make all the other players (you, the ex-wife, his mom and his daughter) follow the rules. And he should back you up 100%, as long as you are acting in accordance with his wishes.