That great. Changing the permissions may work but sometimes the hackers scripts can change them back and then write to the file anyway.
Better protection is always good and it is best to understand how the hacker is gaining access to know what protection you need to improve on. In most cases it is either a vulnerability of the host or the software you have put on the site. If there are no back-doors or vulnerabilities in the software you are running on your site (WordPress, Plugins, Themes, etc.) then it could be the host or another site on the host that is letting your site get reinfected. The only thing I can suggest is to switch to another (more secure) host.
I am currently working on a very secure hosting environment for just such a need. The server is up and I am hosting site on it now but I have not opened it up to the public yet. It will be about $12.75 per site per month, probably more than you are paying now but more secure too
If you are interested in hosting on my new server just send me an email and I can help you make the switch.
Otherwise, best of luck to you and Aloha,