WordPress website security clean and check of persistant Malicious Files

We have a current clients wordpress website that has been hacked about a month ago. We have cleaned and secured the site repretedly however all our attempts to clean malicious files from the site continue to fail. only a few days ago more mailcious script was embedded. We dont know how this is happening. We need a security expert to clean all malicious scripts from the site immediately and set up a secure site which will no longer allow this type of hacking.

We also need you to show the before and after proving that the site is safe from these bad file.

More details:

Is this project for business or personal use? For an existing business
What information should successful freelancers include in their application? Experience
Do you have any specific deadline for completing the project? Immediately or ASAP