Hi Sir/Ma'am,
As per your description above, we understand that you are experiencing recurring security incidents on your WordPress website. Attacks on WordPress websites are very common as it is the most popular CMS. It is open source and most of the plugins/themes provided for various uses are easily targeted for vulnerabilities.
There can be two reason why you are encountering recurring attacks, 1. The attackers have uploaded shells on the website installation which grants them access every time you remove one or clean the site. 2. There are easily exploitable loopholes on your sites which needs to be fixed in order to stop the incident.
We are a bunch of security experts/developers having around 8 years of experience in securing websites. We can assure you that the with our efforts your website will be completely secure. Our team would need to do the following steps to complete eliminate this problem :
1. Clean the server for any malicious files and backdoors.
2. Test the website and fix all security issues.
3. Harden the security and provide easy ways to maintain the achieved standard of security so that you do not face this problem in future.