To talk to only one player, use <tt>/tell</tt> or <tt>/t</tt>. For example:
/tell [Player Name] Hello, can you help me? /t [Player Name] Hello, can you help me?
To ignore somebody, use <tt>/ignore</tt>, followed by their name. Unignoring is accomplished the same way: <tt>/ignore</tt> (without a name) shows a list of ignored players (if any).