How to delete recently used files under ubuntu

Under your ububtu home directory  type
$locate recently


/home/username/.icedtea/cache/recently_used
/home/username/.local/share/recently-used.xbel
$vim /home/username/.local/share/recently-used.xbel

search  the string and delete the  contained <bookmark> tag  

means start and end tag.
<bookmark > ... </bookmark>


ex:


<bookmark href="file:///home/username/eclipses" added="2012-01-19T07:4
4:34Z" modified="2012-01-19T07:44:34Z" visited="2012-01-19T07:44:35Z">
    <info>
      <metadata owner="http://freedesktop.org">
        <mime:mime-type type="inode/directory"/>
        <bookmark:applications>
          <bookmark:application name="File Roller" exec="&apos;file-roller %u&ap
os;" modified="2012-01-19T07:44:34Z" count="1"/>
        </bookmark:applications>
      </metadata>
    </info>
  </bookmark>