To remove checkboxes from newsletters subscription box:

1) Download this file from your server and open in notepad:

content/skins/flat/javascript/validators.js

2) In this file, find lines 180 – 184. They will read:
 
      if((!frm.elements["email_updates"].checked) && (!frm.elements["email_newsletters"].checked)){
                  alert(msg_choose_min_1checkbox);
                  frm.elements["email_updates"].focus();
                  return false;
      }

3) Comment them by adding slashes before each of the lines. The result will be:

//if((!frm.elements["email_updates"].checked) && (!frm.elements["email_newsletters"].checked)){
      //          alert(msg_choose_min_1checkbox);
      //          frm.elements["email_updates"].focus();
      //          return false;
      //}

4) Save the file and upload back to your server.

Those few steps (1-4) will turn off javascript validation.

5) Download from your server and open in notepad the file 

content/skins/flat/boxes/box_subscribe.html

6) In this file find lines 11 – 12. They are :

 <input type="checkbox" checked="checked" value="Yes" name="email_updates" id="id_email_updates"/> <label for="id_email_updates">{$msg.newsletters.subscribe_updates}</label><br />
 <input type="checkbox" checked="checked" value="Yes" name="email_newsletters" id="id_email_newsletter"/> <label for="id_email_newsletter">{$msg.newsletters.subscribe_newsletter}</label>

7) replace these lines with
<input type="hidden" checked="checked" value="Yes" name="email_updates" id="id_email_updates"/>
<input type="hidden" checked="checked" value="Yes" name="email_updates" id="id_email_ newsletter "/>

8) Save this file and upload back to your server.

Those few steps (5-8) will hide the boxes on your site