To unsubscribe from a mailinglist, simply send a message with the word (you guessed it :-) "unsubscribe spacenet" in the body of the message to majordomo@hazmat.com, with the Subject: field blank.
In the event of an address change, it would probably be the wisest to first send an unsubscribe for the old address (this must be done from the old address), and then a new subscribe to the new address.
Most (un)subscription requests are processed automatically without human intervention.
Do not send multiple (un)subscription or info requests in one mail. Only one will be processed per mail.
I intentionally did not include "mailto:" tags above- One of the basic requirements for membership is being able to send mail. Hopefully you won't consider this too tough.