Just keep the weight empty or zero.
If it is actually used in your store, you should alter the "skin/common_files/customer/main/product_details.tpl" and comment out the code like this:
Quote:
{* if $product.weight ne "0.00" or $variants ne ''}
<tr id="product_weight_box"{if $product.weight eq '0.00'} style="display: none;"{/if}>
<td class="property-name">{$lng.lbl_weight}</td>
<td class="property-value" colspan="2">
<span id="product_weight">{$product.weight|formatprice}< /span> {$config.General.weight_symbol}
</td>
</tr>
{/if *}
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http://help.qtmsoft.com/index.php?title=X-Cart:Editing_Skin_Files