The perimeter you create is based on your own behaviors, and not that of others.
1. Two Degrees of Separation; be polite, but do not have contact with anyone who has contact with the mother (except your kid, obviously). It should include those parents at the school, as well as businesses - the banker, the butcher, the baker, the friendly clerk in the store. If they talk to the mother, you don't talk to them. The purpose of this is to stop any back-channel communications, no matter how innocent.
2. Get off Facebook; it's a giant role-playing gossip game in which women excel and enjoy. You are not seeing what other's are doing, you are seeing the spin they put on their activities. There are secondary elements of information available, such as your "friends" that become sources of information about you. The Facebook phenomenon is called "looking at the facts through a keyhole" - a tiny fragment of information in a post, or a photo, becomes the basis for a construct of an entire scenario by the person peeking through the keyhole. You cannot block someone effectively if they want access, so don't think that's a solution. Disable your account, see if you can live without it, then please consider just deleting it - you will never regret it.
3. Force communication to email only; nothing in-person, no phone calls, no texting. There is a dependence on texting with your generation, but it does not serve well as evidence in the Courts, and it has a much higher rate of noise to signal. Initially, we suggest you respond to any future text with a fake message we call Error 23: Put a pre-written note in your phone and reply to the mother with it;
"Error 23: Message undeliverable, sender blocked by receiver"
Sure, it's fake - but the mother won't know that and it socially engineers her to email. Keep in mind that your phone is your personal and private property, and the mother has no legal right to demand certain services be made available to her - particularly when she has a history of abusing those services. The Judge is not going to bat an eye at your blocking her, since you provide the email avenue which is equally timely on a smart phone. Don't ever accept "in case of emergency", because it doesn't matter. If the mother doesn't fall for the Error 23, then block her from texting.
One more thing very necessary for you to build your wall - stop using NJ. That pejorative evolved here as a compromise when members were using much worse, like words that rhyme with itch. But it's actually most damaging to yourself - and your daughter - when you use it. It is a clear indication that the mother is getting too much space, rent-free, in your head. Give the NJ a rest, refer to her as the mother, or the Ex, but move yourself to a place where you DGAF enough to call her petty names. Would you call your daughter a NJ - it's half her DNA.
Ask any questions about these items, we're happy to help you build those boundaries.
Volenti non fit injuria