If you're being bothered by another player, you can block their chat indefinitely by ignoring them. To ignore a player, double-click their name in the list of players. A new window will open showing information about this player. At the bottom of the window, check the box labeled "Ignore." You will no longer receive chat messages from that player.

If you've accidentally ignored somebody or if you'd like to stop ignoring somebody, simply uncheck the "Ignore" box.