How do I leave this community?

And to let you know why you are receiving those emails, it appears you have chosen to Follow the iPad group, so will get an email from every post there. You can go here, and it probably shows "Following" with a checkmark in the upper right of the screen. Click it to remove the checkmark and you will no longer get emails from new posts there.

iPad - Apple Community

You can certainly use the suggestion by MrHoffman and that will also stop all emails too. Just wanted to let you know the source of those emails and the reason why you are getting them.

Tap the round icon in the upper right, and select your preferences for messages from Apple.

You can select as many or as few, or none, as you want.

Apple doesn’t provide a means to delete a nickname around here, though the hosts can do it.