Hightlight a username by double clicking on it or selecting it with your mouse. Right clicking on the highlighted username will open a menu. Select 'Edit List' from this right click menu, then choose 'Censor' from that list.
The usernames on boards (showing near the clocks) do not need to be highlighted. You can directly right click on the name.
You will follow virtually the same process to remove a player from your censor list, except you will choose 'Remove from Censor List' from the 'Edit List'.
You can also choose from the 'Edit List' the option 'Noplay'.
Censor will eliminate all communication from that player and it will block players challenging you or accepting your seeks. This will have no effect if you play in tournaments, 1-minute, 3-minute, 5-minute, 15-minute, 45-minute or 960 pools.
Noplay will only block players challenging you as explained in censor.
Command Line Method for Censor and Noplay:
Open your Main console. (Menu above: Window > Main Console)
To add a player to your censor list, type: +censor username on the command line under your Main console. Example: +censor Fred
To remove a player to your censor list, type: -censor username in the Main console. Example: -censor Fred
Type: =censor to see the players currently on your censor list.
For noplay you can use a similar system.
For information regarding reporting abuse read "help Complain"