Each email that you get will contain the URL of the message, along with the whole message. You can use that URL to view the message on the web, and from that page, you can then select the Email button, and "Unsubscribe" to stop receiving email.
Note that subscription is different from membership or registration. Subscription applies to only one forum or thread at a time whereas your membership info is available in all forums of HyperNews at pcmcia-cs.sourceforge.net. If you are a frequent contributor to these forums, you should become a member. If you are already a member, you can edit your member info, including your email address.