To block a member, visit his or her profile and click the 'Block' link that appears on the top-right corner of the profile. You can view your 'block list' and unblock members by clicking the 'Who Can't I See?' link under the 'Search' tab. Once you block a member, you can't see that person and they can't see you anywhere on the site.
Please note, on mobile phones, the block link is found under the "Account" section at the bottom of the profile. Expand this section, and the block function will be revealed.