Hello,

This is the function which defines name for purchase order form, you can edit original code or overridethis function:

@api.multi
@api.depends('name', 'partner_ref')
def name_get(self):
    result = []
    for po in self:
        name = po.name
        if po.partner_ref:
            name += ' ('+po.partner_ref+')'
        if po.amount_total:
            name += ': ' + formatLang(self.env, po.amount_total, currency_obj=po.currency_id)
        result.append((po.id, name))
    return result

This is available in addons/ purchase/ models/ purchase.py

If you don't want the amount you can delete these two lines

if po.amount_total:
    name += ': ' + formatLang(self.env, po.amount_total, currency_obj=po.currency_id)

i.e, your final code should look like:

@api.multi
@api.depends('name', 'partner_ref')
def name_get(self):
    result = []
    for po in self:
        name = po.name
        if po.partner_ref:
            name += ' ('+po.partner_ref+')'
        result.append((po.id, name))
    return result