KyleMassacre's " !=" or you can use of "<>" bcz both having same mean not equal to
$q=mysql_query("SELECT im.*, i.*, u.*,it.* FROM itemmarket im LEFT JOIN items i ON im.imITEM=i.itmid LEFT JOIN users u ON u.userid=im.imADDER AND i.itemtypeid <> "item id goes here");