Hi,

I think another solution is to define an unlink method in that class. Here I think, you need to inherit the account.invoice class and define the unlink method.

for eg:

class AccountInvoice(models.Model):
    _inherit = "account.invoice"

    @api.multi
    def unlink(self):
        for rec in self:
            if rec.type == 'out_refund':
                raise UserError(_('You cannot delete these type of invoices!'))
        return super(AccountInvoice, self).unlink()

This will allow deletion of records only after checking the condition. 

Hope this helps.