After you logged into Select, click on Profile in the top right corner. Then click on Account on the left of the screen and uncheck all the checkboxes. Finally, click on Save. See the picture below for an illustration.
After you have logged into Select, click on Company Profile in the upper right-hand corner. Then click on Contacts on the left of the screen. Make a selection from the contacts by clicking on the pencil. In the drop-down menu, uncheck all the checkboxes under the e-mail preferences. Finally, click on Edit. See the picture below for an illustration.
Are you still bothered by messages from HeadFirst via e-mail? Send an e-mail to the Servicedesk via email@example.com.