oscommerce site was hacked, need someone help

The people always changed our site code [login to view URL] page, when we changed it back, the people change it back again, please help if you are experienced, and we'll pay with mistone payment, we'll released payment if the people not back in 25 days, we paid for this service last time, but same problem happened, so we want you can guranteen this problem will not happen in 25 days. please only experienced and well kill bidder bid on this project.