It is not possible for a journalist to directly block your brand or organization on the system, however they are able to block the sectors they do not wish to receive press releases from. If a journalist blocks all of the sectors you have defined for your brand in the Brand Settings section (you can define a maximum of 3 sectors for a brand), then that journalist will not receive the mails you send in relation to your brand.
You can view the journalists who block the sector of your brand on the reports page of your press release. The red symbol right in front of the journalist’s name shows that your bulletin is not sent to that person due to the journalist’s preferences.