It looks like you're using the Mobile site to access the message board, so you'll have to change it to Desktop version (Scroll down the page and you'll see Desktop at the bottom). When you've done that, click on the name of the person you want to block. You'll see their profile and on the far right you should see a small cog wheel with an arrow. Tap that and you'll see 'Block Member'. Once you've done that, scroll down to the bottom of the screen and tap of 'Switch to Mobile Version'.
Although this will block initial posts, you will see any of their posts that are quoted by other members.