You can choose to leave a Mighty Network you belong to by going to your Profile Image > Personal Settings > Account > Deactivate My Account (Leave This Network, on Mobile).
From here, if you choose to Leave this Network you will instantly leave the network and you'll stop receiving notifications. However, keep in mind that when you leave a network, your activity (such as any comments or posts that you added) will still be visible to other members.
You can also permanently delete your account on the same menu by using the link found under the text: "To permanently delete your account, click here."
Here's what it looks like on mobile:
And here's what it looks like on the web:
If you're a member of a Mighty Network where you pay a monthly or annual subscription to get access, your account will automatically be cancelled at the end of the current billing period when you choose to leave a network.
If you decide to re-join that Mighty Network in the future, you can always log in again with the same email address or login credentials that you used before.
If you would instead prefer to permanently delete your account and remove all of your activity and posts from that network, click the option to permanently delete your account link at the bottom of the page when you're signed in on your web browser (Chrome, Safari, etc.).