Hide all payment methods if none needed.

Block Class

<?php

class Your_Custom_Block_Rewrite extends Mage_Checkout_Block_Onepage_Payment_Methods
{


    protected function _canUseMethod($method)
    {
        $total = $this->getQuote()->getShippingAddress()->getBaseGrandTotal();
        if((float)$total > 0){return parent::_canUseMethod($method);}
        if($method->getCode() != 'free'){
            return false;
        }
        return parent::_canUseMethod($method);
    }

}

config.xml

<?xml version="1.0"?>
<config>
    <modules>
        <Your_Custom>
			<version>0.1.0</version>
        </Your_Custom>
    </modules>
    <global>
        <blocks>
            <custom>
                <class>Your_Custom_Block</class>
            </custom>
            <checkout>
                <rewrite>
                    <onepage_payment_methods>Your_Custom_Block_Rewrite</onepage_payment_methods>
                </rewrite>
            </checkout>
        </blocks>
	</global>
</config>