1) check the logs and see how they are doing this
2) Which theme are you using? Is it from a known provider or any unknown theme provider? Try changing the theme
3) Try to revoke the permissions..
General advice:
http://ww.wp.xz.cn/support/topic/268083#post-1065779
If this keep on happening, you might want to contact your hosting provider. Just because your site has been hacked doesn’t mean that your site is the problem. A server is only as strong as it’s weakest point and that weak point could be anywhere on the server.
Looks like you fixed it, I just went over your HTML and did not see anything fishy…
What was the cure?