I don't know which mail server you have, but it needs to be set to reject emails sent to accounts that don't exist. You can check that with the following commands. Assuming your mail server for yourdomain.com is at 10.20.30.41:
telnet 10.20.30.41 25
HELO pharma.ca
MAIL FROM:<med_pills70@pharma.ca>
RCPT TO:<sdfghjkltyu@yourdomain.com>
QUIT
If you have the correct setting, the response to the RCPT TO command will have been 550. If you don't get 550, you will need help to configure your mail server.
On the 'Antispam' tab, make sure that *@pharma.ca is blacklisted.
Finally, let's delete most of the unwanted emails using Mail Manager. First, delete all quarantined emails older than a month. Next, in the 'Sender/Rcpt/Subject substring' box, put @pharma.ca and select "1000 entries per page." Now, page by page, use the check box at the top of the page to select all of the messages on the page, select "Delete" at the bottom and press [Go].