You can delete the alias. There is no way to prevent such 'sharing' of
your email address. Email addresses simply cannot be secured - only
kept secret, and, when compromised, abandoned.. you can't password
your email address (only your mailbox) - this realization was the
inspiration for E4ward.com: aliases are essentially passwords.
Unfortunately, if you need to keep hostmaster it cannot be blocked
using E4ward.com. A few spammers exploit the need for such addresses
to exist in a domain. However, most seem to realize that that kind of
attention is not in their best interest.
e.g. I never see spam addressed to or from 'abuse' :)