This kind of thing is almost impossible to block because, as you pointed out, the e-mail address is almost always forged. I have hundreds of e-mails blocked on my account. That being said...
How to block someone via the Messages on the iPhone or iPad
Launch the Messages app from the Home screen of your iPhone or iPad.
Tap the conversation of the person ...
It appears to be a way to copy text from one text field to another. In the messages app I noted that if I drag the text from the message area to the "To" box and wait a short while the plus mark on the dragging text will briefly disappear then reappear. If I let go the text after it has done this it will be added to the To box. I'm hence assuming that this ...
When you click on the word like that it is selected. You can drag the word around at that point. If you select some text and drag it around you'll find that it does the same thing. You'll have to select some bigger text so you can click the middle of it and not change the selection.
The upward pointing arrow in green circle indicates that the message will be send as SMS and not as an iMessage. The name/contact of recipient must be green too.
For iMessage the name/contact would be blue.
Contact shown in blue, registered in iMessage:
Contact shown in green, not-registered in iMessage, will be sent an SMS: