Unfriend her and block her. You can also block her on Messenger. To prevent her creating a new account and accessing you that way, you might want to lock down your profile to be private so that only friends can access it. Consider as well what you want mutual Facebook friends to know, given that they might feed information back to her. One option is to keep your current Facebook as your official account and post nothing important on it, and set up a new account for your real friends.

Block her. Unfollow her. If at all possible set your account to private. However, that possibly defeats the purpose of Instagram, depending on what you’re using it for.
Also, she can still follow you with a new account, or there are ways to follow you without an account at all if she’s tech-savvy enough to figure them out.

Here’s how to block someone on Twitter. You can also mute someone and here’s how to do that. Muting means you’ll just never see their comments, but they won’t realise it, so they’ll be just shouting into the void.
You can block her Twitter account from following you, but unless you lock your account to be available to followers only, she will still be able to read your tweets. Again, you need to decide what is most important to you regarding being able to post publicly on the one hand, and keep her in ignorance about your life on the other.

Here’s how to block on Snapchat.
Here’s now to control your privacy settings on Snapchat.

Here’s how to block someone from commenting on YouTube. It looks like they’ll have had to have made at least one previous comment first.
And here’s how to make your channel private. Again, this is probably defeating the purpose of a YouTube channel, but at least you know it’s possible.

LinkedIn is a complicated one as its sole purpose is networking, so it’s hard to make that narcissistic-mother-proof.
There are two kinds of profiles: public, and (relatively) private.
Private profiles are available only to people who sign up to LinkedIn, but anybody can do that. You can block a member from viewing your profile if you know who to block (so if she sets up a new profile herself, you might not realise it’s her and hence not block her).
Public profiles show up in search engines so people can view them without even being logged in, just by searching your name. You can edit your public profile to show more or less information. The minimum you can show is: name, number of connections, industry and region. Also, it can take several months for the search engine results to reflect the changes you make, and this is out of LinkedIn’s hands.

You have a lot of control over how private or public your content is. And here’s how to block someone.