1. Way: Use the Backend

• Go to Settings > Technical > Actions > Window Actions. (Technical Features must be activated.)
• Search for Opportunities as Action Name and edit it. (Maybe there are more than one entry. Sorry, you have to try which of them is the right one.)
• Remove kanban in View Mode-List
• Remove the kanban view from the view list (at the buttom of the form).

2. Way: Code it

The Window Action your are looking for is a record in crm_lead_menu.xml. To customize the Window Action you can simply override the corresponding record, e.g. in a new module.

Write a xml-file in your module (e.g. crm_lead_menu.xml) with the following code:

<?xml version="1.0" encoding="UTF-8"?>
<openerp>
    <data>
        <!-- remove kanban view from view mode list -->
        <record model="ir.actions.act_window" id="crm.crm_case_category_act_oppor11">
            <field name="view_mode">tree,graph,form,calendar</field>
        </record>

        <!-- create a dummy window action -->
        <record model="ir.actions.act_window" id="dummy_or_whatever_you_want">
            <field name="name">Dummy</field>
            <field name="res_model">crm.lead</field>
            <field name="view_mode">kanban</field>
            <field name="context">{}</field>
        </record>

        <!-- connect kanban view action to dummy window action -->
        <record model="ir.actions.act_window.view" id="crm.action_crm_tag_kanban_view_oppor11">
            <field name="act_window_id" ref="dummy_or_whatever_you_want"/>
        </record>
    </data>
</openerp>

Make sure that your module depends on crm and to include your xml-file in the module's __openerp__.py.

Remark: It would be easier to remove the whole record with the id "crm.action_crm_tag_kanban_view_oppor11". However I dont't get this to work. Instead I created a dummy window action (which do nothing) and point the view action to this (dummy) one.

Thats all. Hope it will help you.