Comment out the code that starts with:
<form id="productsSortForm" action="{$request|escape:'htmlall':'UTF-8'}">
<p class="select">
<select id="selectPrductSort">
And ends with:
<label for="selectPrductSort">{l s='Sort by'}</label>
</p>
</form>
To comment it out just put an exclamation mark and two hyphens after the first bracket for that piece of code (e.g. <!--form id...), and two hyphens before the last bracket (e.g. </form--> ).
To see the change on the frontend, log in to the backend of your PrestaShop application, go to the Preferences tab and then to the Performance sub-tab, and set the Force Compile option to Yes. After you edit the product-sort.tpl file, refresh the frontend, and when you make sure the change appears there disable the Force Compile option.