hide_print_button/purchase_view.xml

<?xml version="1.0" encoding="UTF-8"?>
<openerp>
    <data>
        <record id="purchase_order_form_ext" model="ir.ui.view">
            <field name="name">purchase.order.form.ext</field>
            <field name="model">purchase.order</field>
            <field name="inherit_id" ref="purchase.purchase_order_form"/>
            <field name="arch" type="xml">
            	<data>
                    <button name="print_quotation" states="draft" position="attributes">
                    	<attribute name="invisible">True</attribute>
                    </button>
                    <button name="print_quotation" states="sent" position="attributes">
                    	<attribute name="invisible">True</attribute>
                    </button>
                    <button name="wkf_send_rfq" states="draft" position="attributes">
                      <attribute name="invisible">True</attribute>
                    </button>
                    <button name="wkf_send_rfq" states="sent" position="attributes">
                      <attribute name="invisible">True</attribute>
                    </button>

				       </data>
            </field>
        </record>
	</data>
</openerp>