I actually figured that out by using this:
<div class="col-sm-6">
<div class="form-group">
<label for="inputConfigOption{$configoption.id}">{$configoption.optionname}</label>
<select name="configoption[{$configoption.id}]" id="inputConfigOption{$configoption.id}" class="form-control" {if $productinfo.pid eq 21} disabled=""}{/if}{if $productinfo.pid eq 65} disabled=""}{/if}>
{foreach key=num2 item=options from=$configoption.options}
<option value="{$options.id}"{if $configoption.selectedvalue eq $options.id} selected="selected" {/if}>
{$options.name}
</option>
{/foreach}
</select>
</div>
</div>
(I added this part :
{if $productinfo.pid eq 21} disabled=""}{/if}{if $productinfo.pid eq 65} disabled=""}{/if}
