Click on Members (top menu) or on their avatar anywhere it appears.
Inside the Members page, left hand column, see "Find Member"
Enter the Member's Username and press Enter.
A popup will appear with the Member's avatar, name and other information about them.
There will be three buttons on the bottom of the popup, Follow, Ignore, Start conversation.
Exceptions: Moderators and Admin cannot be blocked. (So you have to put up with me).
Also: In your info, under Your Account you have a section called Settings. Under that, you have an option called Ignoring.
If you select Ignoring, you will get a list of anyone you are ignoring (or it will tell you, you are not currently ignoring any members).
There is an option within Ignore that is selected here.
If it is checked, content from anyone you ignore will not appear at all.
If it is not checked, their posts will be hidden with a message indicating the user is ignored. It also will display an option to show the content.
If someone you ignore starts a thread, you will not see the thread, regardless of who else responds.