You can create a rule based on the To alias and the X-Mailer header.
Go to
Settings->
Rules, click
New rule, and use
Code:
If all of the following conditions apply
Any recipient email is exactly <the desired alias>
(Add condition)
Code:
A header called X-Mailer begins with MailChimp Mailer
Under
Action select
Permanently delete the message
Then click
Save