If you want a simple "work-around" that prevents that option displaying in the Product Input Screen, then do the following:
Find the following file, using FTP to bring it back to your local machine for editing:-
admin/includes/modules/product/collect_info.php
(You must use a PLAIN TEXT EDITOR to edit core files!)
Around lines 287 to 290, find the following:-
<tr>
<td class="main"><?php echo TEXT_PRODUCTS_MANUFACTURER; ?></td>
<td class="main"><?php echo zen_draw_separator('pixel_trans.gif', '24', '15') . ' ' . zen_draw_pull_down_menu('manufacturers_id', $manufacturers_array, $pInfo->manufacturers_id); ?></td>
</tr>
COMMENT OUT that section, making sure you put in some identifier as to why it is commented out - this helps you to find it later if you want to re-instate it)
<!-- EDIT BEGIN - REMOVED TO STOP CONFUSION<tr>
<td class="main"><?php echo TEXT_PRODUCTS_MANUFACTURER; ?></td>
<td class="main"><?php echo zen_draw_separator('pixel_trans.gif', '24', '15') . ' ' . zen_draw_pull_down_menu('manufacturers_id', $manufacturers_array, $pInfo->manufacturers_id); ?></td>
</tr> EDIT END -->
Save the changed file.
Make a BACKUP of the original file.
Send your changed file back to the server, to its original location - (there is no override system in ADMIN).