Or you could just comment out the whole line like this:
<!-- <td><a href="${page:mailLink}&style=${styleid}" target="_blank" title="${msg:chat.window.toolbar.mail_history}" onclick="this.newWindow = window.open('${page:mailLink}&style=${styleid}', 'ForwardMail', 'toolbar=0,scrollbars=0,location=0,statusbar=1,menubar=0,width=603,height=254,resizable=0'); if (this.newWindow != null) {this.newWindow.focus();this.newWindow.opener=window;}return false;"><img class="tplimage iemail" src="${mibewroot}/images/free.gif" border="0" alt="Mail "/></a></td> -->