Block wordpress style for my plugin admin page

The admin stylesheets combines rules for everything: tool bar, left menu, footer and some common elements like tables and forms. You cannot disable just a part of it.

If you want to apply your own rules, use a higher specificity in your CSS rules. Normally, all plugin pages use the same wrapper element:

<div class="wrap">
    <!-- page content here -->
</div>

You can add an id attribute here …

<div class="wrap" id="unique_plugin_prefix">
    <!-- page content here -->
</div>

… and use that selector in your styleheet:

#unique_plugin_prefix p 
{
    margin: 1em 0;
}