In most cases I want to ban "@domain" or "name@", but there is no easy way to alter the e-mail address to this when banning from the user administration page.

I would suggest to implement the following:

On selecting "User administration => Overview => Basic tools => Quick tools => Ban by e-mail" add another text field displaying the current e-mail address.
The administrator or moderator can now alter this e-mail for banning.
On submit the contents of this text field will be used for banning instead of the e-mail displayed in the overview section above.

Thanks in advance for checking this issue.

–
Stefan