That means that there's something extra that's still in your hosting account that's adding that code in. If this is not fxing the problem I'd do a complete backup of ever file, then delete EVERYTHING and upload a new copy of WP And your theme and plugins code. Then you can re-upload your wp-content/uploads/ folder after you have checked it all for any corrupted files.
I've seen this many times before on a few customers websites. The infections are not limited to one file or one place. You need ot look through pretty much EVERY folder on your website to find files that are not supposed to be there. One of the "favourites" that I've seen used is "jquery.js.php". That looks like it's meant to be there but it's a fake file that's set up with the exploit code. There could also be exploit code injected into any of the core wordpress files. It's impossible for us to say or know where it is becuse we can't see the files tructure of your site.
If you are not to good with doing this yourself there's a few good companies around that can help you with these tort of hacks. I won't name any here because I'm not going to promote one over another one, but it might be worth the money for you to get someone to look at it that knows what they are looking for and can see what you are missing there.