How can I delete my account of Realm of the Mad God?
As it currently stands you can not do this.
If you are looking to free up an email address so you can use it to create a new account, you can email them. The link to do so is in the comment below by @MarkTrapp.
There is no current way to delete a realm, but you can edit it. First, you go to the realms menu, next, you click on the chosen realm, lastly press configure and do what you would like to do. Things you can do are switch from world 1, 2, 3, and mini game. You can also change the map. But to answer your question, Go to configure, players, then delete or add the chosen players.