account-financial-tools/account_constraints/view/account_bank_statement.xml

<?xml version="1.0"?>
<openerp>
    <data>
        <!--
            Since account_constraint forbids the deletion of account moves that
            are linked to a bank statement, we add the possibility to delete them
            from the bank statement itself, to let the user fix erroneous reconciles.
        -->
        <record id="bank_statement_cancel_form_inherit" model="ir.ui.view">
            <field name="name">bank.statement.cancel.form.inherit</field>
            <field name="model">account.bank.statement</field>
            <field name="inherit_id" ref="account.view_bank_statement_form"/>
            <field name="arch" type="xml">
                <xpath expr="//field[@name='bank_account_id']" position="after">
                    <field name="state" invisible="1"/>
                    <field name='account_cancel_installed' invisible="1"/>
                    <button name="cancel" attrs="{'invisible': ['|', ('journal_entry_id', '=', False),('account_cancel_installed','=', True)]}" string="Cancel" type="object" icon="gtk-undo"/>
                </xpath>
            </field>
        </record>
    </data>
</openerp>