To block another user, first navigate to their user profile. Next, click on the button with three vertical dots on the banner image to the right of their name. Click "Block this Member".
To unblock a member, navigate to your account settings by clicking on your photo icon in the upper right corner and click on settings. Navigate to "Blocked Users" at the bottom of the menu, and click the unblock button next to their name.
Note: the Blocked Users section will only be displayed if you have blocked another user.